I've been testing my Canon 550D/T2i with video shot at high ISO and the news isn't great. The Auto ISO function on the camera works excellently to increase the ISO as light falls to maintain the exposure but the high ISO performance falls well below that from the full frame cameras.
I guess it was to be expected that as the Canon 5DII high ISO performance is so good that a crop sensor camera would have difficulty coming close but I was surprised how poor the image quality was at ISO 6400. The realistic maximum ISO that I would recommend using for HD video is 1600 to ensure that the image quality is good enough. If you need to shoot at high ISO than this I would suggest using the Canon 5D Mark2 as that provides good quality up to ISO 6400.
