It's a little of both as I understand it. I shot a little video with a friends 5D II and USM L lens (I don't recall what focal length or aperture, but one of the larger white-bodied telefotos). It focused incredibly fast! That is in part due to the very fast focusing of L lenses (And I believe the USM feature as well) AND the variety of auto-focus sensors the 5D II has along with it's fast processor.
FWIW, I read an article a while back on Canon DSLR's used in the video world. (House M.D.'s producer liked to use 5D MK II's for certain shots, and if he used it for one shot he used it for the entire episode for consistency!) It seems like most of them prefer to either, A) Not focus in the shot (use complicated rigs and stage the actors so that they always remain at the correct 'distance' to remain in focus. Needing only very minor adjustments. Except of course when focusing drastically for 'drama', like focusing in between two actors in the middle of an intense scene!) or B) Modify lenses or use big lens-grip-things in order to make them easy to quickly manually focus. Which I imagine takes a bit of skill.
DSLR's take great videos in certain situations and the lenses can create incredibly sharp video with a neat 'cinema film camera' look. But there are some compromises! Remember your DSLR's JOB is to shoot still images, and the video is just a 'feature'. If your primary goal is shooting video but with occasional photos, then a video camera whose JOB is to shoot video, but can snap stills as a 'feature' is a better solution!
(But I do love the take video on my T1i!)
