defined? behaves completely correct in respect to naming conventions. It's behavior is notable in a different way: It's one of the few places where undefined constants can appear in the code without raising an exception:

  jruby-1.6.8 :089 > defined? Foo::Bar
   => nil 
  jruby-1.6.8 :090 > Foo::Bar
  NameError: uninitialized constant Foo::Bar