I have a new base 15" MBP and mine too has constantly showed it's using the nVidia graphics even when nothing (knowingly) is running.
Looking forward to trying bitninja's terminal command after work to see what, if anything, is using it.
For basic web browsing and email I have felt my battery life hasn't lasted as long as it did initially after I bought it 2 weeks ago. Maybe an app I've installed is constantly triggering the nVidia which would explain the battery life.
Isn't OSX 10.6.4 partly supposed to be focussing on graphics drivers?
I saw someone say that over on the Apple discussion boards, among other things. It can focus on graphics drivers all it wants...but if it doesn't allow us to lock one card on at a time, it's no good in my book.