Canon 50D or 7D
If money is no object I say go for the upgrade. If money is a factor stick with the 50D, you are not missing much still photo wise.
If someone is starting out from scratch and was toying between 50D and 7D then definitley go with the 7D, again if you can afford it.
By the way neither has a full frame sensor, you have go up to the 5D or 1Ds for that
If you shoot in dirty or wet environments, or make poster size prints then consider the 7D.
Unless you have or can buy L lenses, you won't get much benefit from the 7D in terms of print quality.
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Well, with the help of this thread and particularly Ruud's input, I decided not to get a 7D and instead got a 5D Mark II. The 5D MkII kit price breaks and indications that the 5D Mk III might be a year away helped the decision. In fact I got a $750 Canon printer for $250 as part of the deal (from Adorama).
So, I've now shot 225 photos with the 5D MkII. It's a dream. The interface is much like the 50D and 7D but the large bright viewfinder is great for my 69 year-old eyes. The shutter release button seems to know what I'm thinking. It's far better than the ones on the 30D, 40D and 50D that I've owned. It hardly moves down but locks in the AF and takes the shot with no conscious effort on controlling the finger movement. And, talk about low noise. It may not compete with the best Nikons but I've shot some at ISO 1600 and a few at ISO 3200 and LR or CS5 and probably Canon's software clean up the noise with very little softening of the image. You run the slider up until the noise just disappears than back off slightly and you have very little softening.
I never had a 7D in my hands, but can't say enough about the 5D MkII.
If you not need speed shooting 60D or even 550D/600D will be best from "quality/price ratio" - they are cheaper than 7D. All these cameras have same 18MP sensors and Digic IV processor, different is speed p/s.