I have neither academic nor technical experience in what I'll call Timber Framing. I am a neophyte in the area but have been interested for the last two years in TF and have been visiting with companies that specialize in TF. So this is just an educated guess.
I really think they mean the same thing and people use them interchangeably. However I think people are using the words timber framing more frequently today and infact when I talk to old carpenters they seem to use the words post and beam more often. This might have something to do with the guild and the name it holds, thus giving the words timber framing more legitamacy. ????
However, someone who has been around timber framing circles alot longer than I have may be able to distill the sematics and see the fine line between timber framing and post and beam. Just like one can see the difference between a cup and a mug. But then again that can be a matter of interpretation.

Another guess, if I may, is that "timber framing" implies that the timber is part of the frame. Many times, in addition to old carpenters, I hear stick framers call it post and beam. Generallly they are not masters at traditional joinery so if they are asked to put in decorative timbers they are false post and beam and not load bearing (ie not part of the frame, therefore not timber framing)

I'm all out of guesses, good luck

Ian