I was debugging a little problem just today and figured out that defined(Class[‘something’]) would return true if in the current scope, there is a class something.
Example:

class foo {
  notify{'I am class foo': }
}
class bar::foo {
  notify {'I am class bar::foo': }
  if ! defined(Class['foo']) {
    notify {'foo was not declared yet. do it!': }
    include foo
  }
}
include bar::foo

This results in

Notice: I am class bar::foo
Notice: /Stage[main]/Bar::Foo/Notify[I am class bar::foo]/message: defined 'message' as 'I am class bar::foo'

Not quite what I expected. I added some debug statements in the defined function and figured out that he resolved Class[‘foo’] to Class[‘bar::foo’].
After this, It was pretty easy to fix. Also note that you need to add the ‘::’ when including foo too!

class foo {
  notify{'I am class foo': }
}
class bar::foo {
  if ! defined(Class['::foo']) {
    notify {'foo was not declared yet. do it!': }
    include ::foo
  }
}

Notice: foo was not declared yet. do it!
Notice: I am class foo
Notice: I am class bar::foo

HURRAY! So, as a general rule, always ::scope everything where you can ;)