Valve’s done a pretty good job at filmmaking so far without either detracting from or relying on the game’s experience. (The Meet The Team videos, the L4D/2 intros)
I imagine a HL/Portal film would stay separate from the timeline and main plot of the games, a look at the initial GLaDOS incident at Aperture or the 7 hour war, perhaps.
IIRC, the only reason they never let anybody make a HL movie was because all the scripts they got offers with were crap.