WARNING: SPOILERS for those who haven't seen the Avengers movie (all three of you out there).
A lot of people have mentioned this: We never actually see Coulson die in the movie. Nor do we see a body.
He passes out and a medic attends to him, then we hear Nick Fury over the comm saying "Agent Coulson is down."
One of the theories is that Fury needed to galvanize the Avengers into action so he and Coulson faked it.
TV shows based on movies, never follow the movie plot exactly, so it could be a simple TV universe, which would allow Movie charactors to come in without using the actors, who probably don't want to be doing a TV show right now.
They've said (before casting Clark Greg) that this shows universe and the movies were not related..BUT, clearly they wanted to cash in on Colsons (Clark Greg's) popularity and have screwed up what they originally said...really curious to see how this plays out...