I plan to shoot a program in 16:9 with a Canon XL-1. (I've read the posts about the pluses and minuses of using the 16:9 feature of the Canon).

Many of the shots will be short distance. Can I use the Century Optics wide angle adapter with the standard 16x lens and still be in 16:9? Will this adapter significantly degrade the image?

The final program will is for CD-ROM distribution. Should the final 16:9 wide angle image be okay on 800 x 600 resolution (letterboxed)?

I don't see why you couldn't. The 16:9 on the XL1 is an ELECTRONIC
STRETCH, not a true 16:9 anamorphic chip. Since the images is
already capture by the chips I don't see why this wouldn't work.

I would suggest to do some test before you go out to record it
for real. That's always a good thing to do if you have some new
equipment or settings to work with.