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 (RSHIFT+RCOMMAND, RSHIFT+LCOMMAND, LSHIFT+RCOMMAND) 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 L-Shift and L-Command in TextEdit; same problem.