A good friend of mine used to wear this while riding:

Was the funniest thing ever. I'm seriously debating buying one, too.

I'm rocking this at the moment:
http://www.sierratradingpost.com/p/1...f-For-Men.html

And i just saw they're on sale, so I'm going to suggest this product. I wear it in the rain/snow, summer/winter, and it does it's job very well. For winter riding, in temps anywhere from 0 to 20 [F], I wear a standard waffle thermal under a slim jacket (in my case either the Momentum from North Face, or a regulatory fleece from Patagonia), and this on top. Hundo percent warm.