* 7D, 60D, 600D, 550D have exactly the same sensor and processor, and deliver exactly the same image quality in video mode; 5D2 is slightly better, but lacks 720p50/720p60, which you'd use for overcranking (slow motion) (maybe not too important: 720p has awful aliasing/moire, even worse than 1080p); 1D4 is slightly worse than any of the others (except if you shoot with extremely low light)
* 5D2, 60D, 600D, 550D can be modded with Magic Lantern, which is absolutely awesome: custom cropmarks, zebras, live histogram or waveform while recording, peaking, focus aid (zoom-in) while recording, etc; and also manual gain for audio, and increased bitrate, which I've never used; the 7D can't be modded
* 5D2 and 7D have magnessium body with weather sealing; the 60D and 600D have a great swivel screen (which is really, really great for video, just make sure you get a loupe that supports it)
* 5D2 takes the best stills; 7D has the best focusing system; 60D, 600D, 550D lack autofocus microadjustment, which can be a huge issue for stills if you actually want to use AF (which I don't) (there's no autofocus while recording video anyway, except for some magic lantern hack that I've never used)
and anyway, that's all about the canons; right now, sony seems to be doing a bit better: the first tests I've seen from the tiny nex-5n looks awesome; stuff from the nex-7, A65 and A77 could be even better (as in: even less aliasing/moire)