Early 2011 15" Macbook Pro, Lion, no external keyboard or mouse:
Lately I noticed that I can't use left-shift+left-command together; it registers as left-command only. The other three combinations (right-shift+right-command, right-shift+left-command, left-shift+right-command) work properly. I’ve confirmed this in text editors and in the keyboard viewer.
Same problem appears with left-command+left-control.
I’ve shut down all programs, verified no weird keybinds in Universal Access or elsewhere, even installed Lion to a clean external disk and booted from that. The first thing I did was check left-shift and left-command in TextEdit; same problem.