AustinFoodCarts released an iPhone app today that uses your current location to find the closest food cart. It includes a map view and allows you to browse by cuisine and distance; it also includes basic information on each cart. It seems pretty useful, especially for trucks that move around. Also, it's free, which we can't really complain about, and seems to cover carts as far away as Pflugerville.
Screenshot of AustinFoodCarts iPhone app. [Photo: Apple]