Maybe you don't have room for a normal sized bike, or just like the idea of a bicycle that can be carried around under your arm, but either way, you'll get a kick out of the eZee Bikes Quando. When fully expanded, this bike weighs 57 pounds including the NiMH 36V 9AH battery.
In terms of speed, you should be able to get up to 20 mph with a cruising speed of 15mph, which sounds good to me. And the company says it only takes 30 seconds to unfold, though I assume that depends on how savvy a bike unfolder you really are. There are a lot of other wacky folding bikes in case you're curious, but the Quando will run you about $1,300.
eZee Quando - A Folding Electric Bike [Treehugger]