There are quite a few scanners that will draw power from the USB port. Unfortunately they are usually the lower resolution scanners. There are also some mini photo scanners such as the HP1000. Its a great form factor (about the size of a 4x6 photo) but is too proprietary and only a 30-bit scanner. They go for about $30 at CompUSA (http://h10010.www1.hp.com/wwpc/us/en/ho/WF06b/15179-64195-64341-298148-f13-42767-42768-42771.html)

You are really only going to get the level of scan that you would with a higher resolution scanner and I havent seen too many of them that are powered by the USB port. Fortunately there are other options such as a DC/AC converter for your car, or even some of the new cell phones with digital cameras.