I had a banding problem with my 60D, just on some shots & it really started to annoy me.
(As yellow said you must keep to the right Decoding Matrix & Luminance range)
I started to use the DPX file output option on 5DtoRGB - huge files, but amazing quality. I thought that this had solved the banding problem, but it was still noticeable on some shots - mostly white or off-white backgrounds.
For screen grabs use MPEGStreamclip (its free & widely used by editors) & that goes double for converting any footage that comes out of FCP (you just use the simple export setting, NOT export to QT) - QT is a nice toy, but that's all it is!
[u]The Solution:[/u] I am trying out the free 30 day trial of FCPX & guess what? No banding at all! I tried all the shots that had banding in FCP7 & in FCPX the banding was not present at all, just lovely clear footage like i knew i'd shot!
I'm still playing with FCPX, but the more i do, the more i like it & will probably dump FCP7 for DSLR footage completely.
Hope that helps a little