Me and my buddies were talking about the scale app too but I think it's just a pipe dream dude. You'd need some sort of hardware to actually act as a sensor. The touch screen itself wouldn't hack it cause it doesn't respond if you press it too hard, so there must be some sort of weight limit. That would be the ultimate stealth scale though.

What about a scale that is disguised as an ipod, but is also a functioning ipod incase the boys in blue get a little snoopy...