They didn't "exactly" kill Data! In the last movie, (Nemesis) another android that looked like Data was found/made (my memory's a little rusty on this one). The "origional" Data saved Picard's life, but ended up getting killed BUT NOT BEFORE he made a "transfer" of sorts to the "new Data" THEREFORE Data isn't "really" dead - the "new guy" is very much an infant of sorts. This leaves plenty of room for Brent Spinner (the actor) to come back and take the character in a whole new direction.