Here's one method:
1. Use VLC (http://www.videolan.org) to capture the iSight video feed (File > Open Capture Device > Capture > iSight). Position the VLC window where you want it. If desired, set VLC preferences so that the VLC window always floats on top of everything else.
2. Start QuickTime X and begin screen recording.
Alternatives to VLC: iChat, Photobooth, etc., though VLC seems to consume fewer system resources than Apple applications for this purpose.