I remember seeing something on Gizmodo.com that pretty much looked like a bluetooth headset that you would were and it would record constantly and just keep the last 15-30 min and continuously record over the older stuff and it was made just for what you are looking for. It was made so that when something cool (or bad) happened that you would want on video, you would just press a button and it would stop recording and you could just plug it into your computer and get the last 15-30min or however much it was. If I see it again I'll post a link up on here for you.

EDIT:
Found it. Its called the Looxcie LX1. You can get it on amazon for $150 and it records the last 4 hours not just 15 min.
http://www.amazon.com/Looxcie-LX1-Bl.../dp/B004DI7F76
and heres some more info on it.
http://gizmodo.com/5639699/the-looxc...tops-recording