A mobile service I'd actually use. CellWand lets you dial #TAXI on your cellphone, say the name of the city you're in (though why this can't be derived from the cell you've connected to…) and you'll be connected to the first available taxi company's dispatch. Ideally the thing would also identify your location, but that's gotta wait for E911. Still, I hope it can at least give your phone number to the dispatch so you don't have to read it back to them. I have no sense of direction, so my usual MO in a strange city is to just wander around to whereever I'm going, then hail a cab back to the hotel. This works everywhere except the west coast, where cabs are basically nonexistent.
(Thanks, Rick!)