I am currently attending a four-day conference on "The Posthuman" in Rome, which is about to end today. Since the attendees are quite a mixed bunch, terminology is an issue, and especially the question what exactly the difference is between posthumanism and transhumanism has frequently come up. So if you need some quick clarification regarding the matter, here is the brief answer that I would give (borrowing heavily from Donna Haraway) and which I think sums up the difference between the two approaches quite nicely:
Posthumanists want to be cyborgs rather than gods. Transhumanists want to be both - but if they had to choose, they would much rather be gods.