Back in the old days, when I was running Natty (last month), I was able to stretch a single wallpaper over both my monitors in Ubuntu. After I upgraded to Oneiric, it totally killed my old wallpaper, and (more importantly) I can't find any setting that would let me do the same thing again. It's a small thing, but annoying.

It might not be an Xfce issue, as I may have been using Compiz or maybe it has something to do with the NVidia drivers. But I can't find it in any case, and I would appreciate any help anyone could provide.

I have somewhat the similar problem. Are you using gdm by chance? I would try changing your gdm background to something different just to be sure it's that.