I guess this must be new in QuickTime Player 7 as I haven't seen it before, and haven't seen any documentation for it.
You can now move the movie frame-by-frame, backward or forward, via the scroll wheel on a so-equipped mouse. If the movie is already playing, it will enter pause mode and then step frame by frame. Roll the wheel "up" for forward, and "down" for backward.
[robg adds: I thought we'd run something on this already, but I can't find it...]
Mac OS X Hints