Extracted version from gemspec and placed it in its own file. This is consistent with the approach taken by "bundle gem", and is expected by gems such as svenfuchs/gem-release which can be used to bump / tag versions of gems.

railties/lib/rails/generators/rails/plugin_new/plugin_new_generator.rb

@@ -46,6 +46,7 @@ module Rails
     def lib
       template "lib/%name%.rb"
       template "lib/tasks/%name%_tasks.rake"
+      template "lib/%name%/version.rb"
       if full?
         template "lib/%name%/engine.rb"
       end

railties/lib/rails/generators/rails/plugin_new/templates/%name%.gemspec

@@ -1,8 +1,12 @@
-# Provide a simple gemspec so you can easily use your
-# project in your rails apps through git.
+$:.push File.expand_path("../lib", __FILE__)
+
+# Maintain your gem's version:
+require "<%= name %>/version"
+
+# Describe your gem and declare its dependencies:
 Gem::Specification.new do |s|
   s.name        = "<%= name %>"
-  s.version     = "0.0.1"
+  s.version     = <%= camelized %>::VERSION
   s.authors     = ["TODO: Your name"]
   s.email       = ["TODO: Your email"]
   s.homepage    = "TODO"

railties/lib/rails/generators/rails/plugin_new/templates/lib/%name%/version.rb

@@ -0,0 +1,3 @@
+module <%= camelized %>
+  VERSION = "0.0.1"
+end