The buzz on the EOS mailing list is the significant vignetting in the corners seen on wide angle shots at open apertures with the new full-frame Canon EOS 5D. William Coburn was kind enough to show an example using a 20mm lens at f/2.8 versus f/5.6: http://infohost.nmt.edu/~schlake/5d/2point8vs5point6.jpg
Now, some of this is undoubtedly due to the lens itself (most wide-angles suffer from corner darkening at full apertures), but digital sensors are susceptible to more vignetting (or more accurate, light-falloff) with wide-angle lenses* due to their surface microlens design. I decided to hold off buying the 5D until I had gotten more user reports and I'm glad to have done so. Let's see how this shakes out.
* The Epson R-D1 digital rangefinder camera has a software tool to reduce vignetting because of this problem. And people with Photoshop CS/CS2 can correct this in the RAW plugin too.
Update: Mehyar refers me to Michael Reichmann's excellent field report with a pre-production EOS 5D. Michael evaluates the cameras a photographer and not as a tech-geek.