I've got a 17" - it's on Intel for me most of the time. One person who sent me some feedback on gfxCardStatus showed me a way to see which processes are using the NVIDIA card. Run this in a Terminal:
ps cx | grep $(ioreg -l | grep task-list | sed -e 's/(//' | sed -e 's/)//' | awk ' { print $6 }')
That will show you which process(es) are using the 330M at any given time, and grep will complain about not having a pattern if there are no processes using it. I'll be putting this functionality in gfxCardStatus soon, so you can see which processes are using the card right from the menu bar icon.
Thanks, rawd! I love Growl.