As a photographer, I was excited when Canon announced the release of the 5D.  At under $3000, it's a bargain for a full frame digital camera and I was anxious to get my hands on one.  After doing some research, I wasn't convinced that the extra investment over a 20D was justified.  Here's why:

Pixel Density

Because the 20D has smaller pixels (6.4 microns vs. 8.2 microns), the pixel density of the 20D is 24,252 per square mm.  while the pixel density of the 5D id 14,165 per square mm.  What this means is that the 20D captures more detail than the 5D.  The larger sensor size of the 5D will theoretically yield better enlargements over 8x10, much depends on the lens and other factors.  There is a formula called the "Zeiss Circle of Confusion" that is generally relied upon to determine the enlargement capacity of a given sensor or format.  You can google "Zeiss CoC" if you want a more indepth understanding of the formula and it's meaning.

Lens Factor

Speaking of the lens, the smaller image sensor of the 20D has an advantage over the 5D.  Except for EFS lenses specifically designed for the APC sized sensor, most other compatible lenses are designed for 35mm format, or the larger sensor of the 5D.  What this means is that the smaller sensor uses the "sweet spot" of the lens - the center portion.  All lenses tend to give poorer performance at the outside of their coverage.

Crop Factor

The Crop Factor is often misunderstood.  Basically, if you take a full frame image and "cut out" the size of the smaller sensor the resulting image is what the 20D sensor is seeing.  In order to fill a full size sensor frame with the exact same image size captured on the smaller sensor with a 50mm lens,  you would need a lens with a focal length of 1.6 times the focal length, or 80mm.

Applying this knowledge to the question, we discover two things.  A lens of with a focal length of 200mm on a 20D, for example, would yield the equivelant of a 320mm lens on a 5D  (200x1.6).  This becomes most important to the issue at longer lens lengths.   At a set distance with the same lens, the full size image from the 5D would have to be cropped and enlarged to give the same result as the 20D.  The 20D's higher pixel density will reveal more detail than the 5D.  Of course if you have access to a longer lens or are able to more closer to the subject, the 5D will surpass the 20D.

Conclusion

While the 5D is an outstanding camera, it may not have to replace your 20D.  If you shoot a lot with telephoto, the 20D may actually have an edge on the 5D.  If you need to make huge enlargements, the 5D (or better yet the 1Ds MK II) will yield better results.

When I put all the factors together, I conclude that the 5D is not a necessary upgrade at this time.  In fact, I think I'm going to buy another 20D and use the extra money to invest in better lenses!

I know this is by no means a definitve comparison between these great cameras, just my observations from research and some limited hands on testing.  I hope this is of assistance.  Feel free to add your comments.