Index: boost/program_options/detail/value_semantic.hpp
===================================================================
--- boost/program_options/detail/value_semantic.hpp	(revision 38498)
+++ boost/program_options/detail/value_semantic.hpp	(working copy)
@@ -16,7 +16,13 @@
     std::string
     typed_value<T, charT>::name() const
     {
-        if (!m_default_value.empty() && !m_default_value_as_text.empty()) {
+        if (!m_implicit_value.empty() && !m_implicit_value_as_text.empty()) {
+            std::string msg = "[=arg(=" + m_implicit_value_as_text + ")]";
+            if (!m_default_value.empty() && !m_default_value_as_text.empty())
+                msg += " (=" + m_default_value_as_text + ")";
+            return msg;
+        }
+        else if (!m_default_value.empty() && !m_default_value_as_text.empty()) {
             return arg + " (=" + m_default_value_as_text + ")";
         } else {
             return arg;
@@ -155,7 +161,13 @@
     xparse(boost::any& value_store, 
            const std::vector<std::basic_string<charT> >& new_tokens) const
     {
-        validate(value_store, new_tokens, (T*)0, 0);
+        // If no tokens were given, and the option accepts an implicit
+        // value, then assign the implicit value as the stored value;
+        // otherwise, validate the user-provided token(s).
+        if (new_tokens.empty() && !m_implicit_value.empty())
+            value_store = m_implicit_value;
+        else
+            validate(value_store, new_tokens, (T*)0, 0);
     }
 
     template<class T>
Index: boost/program_options/value_semantic.hpp
===================================================================
--- boost/program_options/value_semantic.hpp	(revision 38498)
+++ boost/program_options/value_semantic.hpp	(working copy)
@@ -204,6 +204,38 @@
             return this;
         }
 
+        /** Specifies an implicit value, which will be used
+            if the option is given, but without an adjacent value.
+            Using this implies that an explicit value is optional, but if
+            given, must be strictly adjacent to the option, i.e.: '-ovalue'
+            or '--option=value'.  Giving '-o' or '--option' will cause the
+            implicit value to be applied.
+        */
+        typed_value* implicit_value(const T &v)
+        {
+            m_implicit_value = boost::any(v);
+            m_implicit_value_as_text =
+                boost::lexical_cast<std::string>(v);
+            return this;
+        }
+
+        /** Specifies an implicit value, which will be used
+            if the option is given, but without an adjacent value.
+            Using this implies that an explicit value is optional, but if
+            given, must be strictly adjacent to the option, i.e.: '-ovalue'
+            or '--option=value'.  Giving '-o' or '--option' will cause the
+            implicit value to be applied.
+            Unlike the above overload, the type 'T' need not provide
+            operator<< for ostream, but textual representation of default
+            value must be provided by the user.
+        */
+        typed_value* implicit_value(const T &v, const std::string& textual)
+        {
+            m_implicit_value = boost::any(v);
+            m_implicit_value_as_text = textual;
+            return this;
+        }
+
         /** Specifies a function to be called when the final value
             is determined. */
         typed_value* notifier(function1<void, const T&> f)
@@ -243,7 +275,7 @@
 
         unsigned min_tokens() const
         {
-            if (m_zero_tokens) {
+            if (m_zero_tokens || !m_implicit_value.empty()) {
                 return 0;
             } else {
                 return 1;
@@ -301,6 +333,8 @@
         // as boost::optional to avoid unnecessary instantiations.
         boost::any m_default_value;
         std::string m_default_value_as_text;
+        boost::any m_implicit_value;
+        std::string m_implicit_value_as_text;
         bool m_composing, m_implicit, m_multitoken, m_zero_tokens;
         boost::function1<void, const T&> m_notifier;
     };
Index: libs/program_options/test/variable_map_test.cpp
===================================================================
--- libs/program_options/test/variable_map_test.cpp	(revision 38498)
+++ libs/program_options/test/variable_map_test.cpp	(working copy)
@@ -102,6 +102,25 @@
     BOOST_CHECK(vm3["vee"].as<string>() == "42");
     BOOST_CHECK(vm3["voo"].as<string>() == "1");
     BOOST_CHECK(vm3["iii"].as<int>() == 123);
+
+    options_description desc3;
+    desc3.add_options()
+    ("imp", po::value<int>()->implicit_value(100))
+    ("iim", po::value<int>()->implicit_value(200)->default_value(201))
+    ("mmp,m", po::value<int>()->implicit_value(123)->default_value(124))
+    ;
+    char* cmdline6_[] = {  "--imp=1 -m" };
+    vector<string> cmdline6 = sv(cmdline6_,
+                                 sizeof(cmdline6_)/sizeof(cmdline6_[0]));
+    parsed_options a6 = command_line_parser(cmdline6).options(desc3).run();
+
+    variables_map vm4;
+    store(a6, vm4);
+    notify(vm4);
+    BOOST_REQUIRE(vm4.size() == 3);
+    BOOST_CHECK(vm4["imp"].as<int>() == 1);
+    BOOST_CHECK(vm4["iim"].as<int>() == 201);
+    BOOST_CHECK(vm4["mmp"].as<int>() == 123);
 }
 
 int stored_value;
Index: libs/program_options/src/cmdline.cpp
===================================================================
--- libs/program_options/src/cmdline.cpp	(revision 38498)
+++ libs/program_options/src/cmdline.cpp	(working copy)
@@ -329,6 +329,14 @@
             
             max_tokens -= opt.value.size();
 
+            // A value is optional if min_tokens == 0, but max_tokens > 0.
+            // If a value is optional, it must appear in opt.value (because
+            // it was 'adjacent'.  Otherwise, remove the expectation of a
+            // non-adjacent value.  (For now, we just check max_tokens == 1,
+            // as there is no current support for max_tokens>1)
+            if (min_tokens == 0 && max_tokens == 1 && opt.value.empty())
+                --max_tokens;
+
             // Everything's OK, move the values to the result.            
             for(;!other_tokens.empty() && max_tokens--; ) {
                 opt.value.push_back(other_tokens[0]);
Index: libs/program_options/example/options_description.cpp
===================================================================
--- libs/program_options/example/options_description.cpp	(revision 38498)
+++ libs/program_options/example/options_description.cpp	(working copy)
@@ -26,11 +26,17 @@
 {
     try {
         int opt;
+        int portnum;
         po::options_description desc("Allowed options");
         desc.add_options()
             ("help", "produce help message")
             ("optimization", po::value<int>(&opt)->default_value(10), 
                   "optimization level")
+            ("verbose,v", po::value<int>()->implicit_value(1),
+                  "enable verbosity (optionally specify level)")
+            ("listen,l", po::value<int>(&portnum)->implicit_value(1001)
+                  ->default_value(0,"no"),
+                  "listen on a port.")
             ("include-path,I", po::value< vector<string> >(), 
                   "include path")
             ("input-file", po::value< vector<string> >(), "input file")
@@ -62,7 +68,14 @@
                  << vm["input-file"].as< vector<string> >() << "\n";
         }
 
+        if (vm.count("verbose")) {
+            cout << "Verbosity enabled.  Level is " << vm["verbose"].as<int>()
+                 << "\n";
+        }
+
         cout << "Optimization level is " << opt << "\n";                
+
+        cout << "Listen port is " << portnum << "\n";                
     }
     catch(exception& e)
     {