While at $1,700.00 Nikon will surely sell every D200 they can produce, I don't think you'll see Canon 20D sales dip much. Especially wiht Canon recent rebates, plus the fact that so many pros and semi-pros have used this model to produce stunning images.
As far as comparing to the Canon 5D, maybe......At 12.8 MP plus fulll frame, same improved bigger viewfinder, etc. the Cano 5D will still sell like hotcakes...Maybe not in the UW world as much, bu in general Canon will sell mucho' 5Ds.....
I might expect many EOS 1D series shooters to consider the 5D camera as a lighter weight alternative, lower cost professional tool, too.
YMMV
David Haas
Lowly little Rebel XT shooter 