Dan Achatz's recent discussion thread in the PFRE discussion group was very insightful for me. Dan sounds like he is struggling to keep the sensor of his new Canon 5D Mk II free of dust. I feel his pain. I've fought the "dust on the sensor" battle since 2003 when I got my 1Ds. For the first couple of years I got really worked up about keeping the sensor dust free. I hardly ever changed lenses. Now I've gotten to the point that I just live with a little dust on the sensor and spot the images unless it gets really obnoxious.
For those you that haven't struggled with the full frame dust problem, full frame sensors are famous for being dust magnets. You have to clean them regularly. The only way you keep dust off the sensor is don't remove the lens. Of course the 5D Mk II has a bunch of new features to reduce, repel and remove dust. I assumed that all this wizzy new technology on new full frame cameras had the dust problem under control- Apparently not.
When I gave Dan my "live with it" philosophy he pointed out that when you are shooting HD video, like the 5D Mk II does, you can't live with it! How do you spot a HD video? HD video is notorious for showing every little detail, like dust spots. Wow, this rattled my cage. I was really warming up to the new 5D. Mostly to be able to shoot HD video with high quality interchangeable lenses. I'd never thought about the implications of shooting HD video, that shows every spec of dust, with a sensor that's hard to clean.
Don't get me wrong, the 5D Mk II has some great new technology. Even without the HD video feature, it's well worth the money as a still camera. But I think if you are getting it to shoot HD video it sounds like you should be prepared to spend some serious time sensor cleaning or just put a lens on it and keep it on.