I finally figured this out (or found another post about it on another message board)... I was using a Yahoo Widget called Month Calendar by Harry Whitfield. It was hijacking the ctrl + space keystroke. I went into the prefs and cleared it out and all is good.
It had stopped for a while and then it started happening again. Do you use Logitech SetPoint on your system? I thought it might have something to do with that, but I'm not completely sure.