As we wait out the coming apocalypse, might as well have fun with it. A friend of mine has built herself one of these, and once the engine was broken in it topped out at about 45 mph. That's pretty fast for a bike, and calls for a few safety precautions not normally needed for bikes. It's a bit of a fabrication challenge if you put it together yourself, and it sounds like a weed-whacker when it goes by. You ought to get a sturdier helmet than the normal type for bicycles, and the best tires available since a blowout at 45 mph on a bicycle really sucks.
She commutes about 9.5 miles one way to work, over a large and steep hill at one point, and she buys about a gallon of gas a month. It's a fairly low-traffic area, but legal to take anywhere as it's under the DOT's 50cc cutoff for engines requiring registration.
http://www.staton-inc.com/photo_gallery.asp 