Michele,
It does appear to be a "focus" issue, as Mike has suggested. This is something I cannot do on my digital camera, but on my SLR you can lock the focus, recompose the shot, then take the picture. I'm wondering if this is a feature on the next-generation 5+ megapixel cameras described by Morgan Weistling in the
"Considering Buying a Digital Camera" thread. It should definitely be an option on an auto-focus camera; you could set up a color bar or a grey card, focus on that "absolute" edge, recompose and shoot.
I'm curious because I use the same setup you've described, and have the same focus issues.