In the category of "things that you can't actually buy right now", I think this looks like the coolest thing ever -
http://www.bikerumor.com/2010/10/12/...ling-computer/
Cerevellum Unveils Awesome Rear View Video Camera Cycling Computer



More Details:
http://www.bikerumor.com/2010/10/12/...ling-computer/

Unfortunately it's apparently in the "We're trying to get funding for it" stage, and it costs at least $250, and you'd have to recharge the battery on it.

Still...I've never been really happy with any bicycle mirrors I've tried. Even car mirrors seemed to small until my latest cars which has some sizeable mirrors on it. Not sure I'd be happy with anything smaller than a mirror of that size, being that on my bike I'm in the same conditions I'm in in my car.

But if the lcd was viewable in direct sunlight, the display was big enough, and the camera was a decent resolution, this would be awesome. I'd feel a hell of a lot safer biking on the road.