Personally "meet someone" means something more general
And "meet up with someone" means a specific time.
( Pickle Fairy is clumsily dancing away this fear, Mon 11 Aug 2008, 18:14, archived)

yes, this is confirmed by the dictionary
to meet up means it was pre arranged as well
( Enigmatic ™                 Voice No More                  ©, Mon 11 Aug 2008, 18:16, archived)

I hear that Todd refuses to fill in forms
she will only fill out forms.
(rnuk http://flickr.com/rnuk, Mon 11 Aug 2008, 18:28, archived)