Cyclists love to bitch about the unnecessary weight involved with carrying a quality u-lock, but those same riders also really hate having their bikes stolen. So if you could incorporate a red light into a bike lock, saving a little weight and adding functionality, that really wouldn't be a bad thing. Hiplok FLX is a £29.99 ($58) cable lock that claims to do exactly that. It builds a 10-lumen LED into a clip-on cable lock, which is designed to be hooked onto your belt or bag.
It's not the most serious of tools -- a 10-lumen LED is only good for helping drivers see you, and the thin cable isn't going to fare well against bolt cutters, let alone serious bike thieves.
But the principle is sound -- the most comfortable way to carry a serious u-lock is threaded through your belt, and there's lots of unused real estate in the heavy bottom bar that could easily hide an LED. Smells like a Kickstarter in waiting to me.