[update: see also a MB Denoiser review by Steve Douglas on Ken Stone's FCP]
Update: There are a number of other noise filters available, like the Remove Grain effect built into After Effects (see video below by Matthew Borgatti), or DE:Noise, Neat Video (AdobeTV demo), Video Purifier and so forth. On the AE-list (Nov 20, 2010), Brian Behm commented on noise reduction for keying:
"I think you'll find there isn't a definitive answer out there because there's no definitive answer. When I'm keying footage shot on a T2i (fairly similar to the 7D video wise) I sometimes need to denoise and I sometimes don't.
In general though, when I do denoise it's early in the process before the key and then afterwards I'll go back and grain sample the original footage and add it back to my composited scene. Generally you're denoising as a way to assist in pulling a cleaner key so it makes sense that you'd do that before you pull the key."