We are beginning work on a senior project for my Computer Tech class and I recommended a quadro-copter. One thing that we couldn't quite figure out was how to use the arduino board to control the resistance to the individual motors. Is there some serial device or anything we can do to solve this problem?
Now a days almost all electric motors used in quad type aircraft use brush-less DC motors that have to each have their own properly sized electronic speed controller (ESC). Esc are interfaced to a controller just like any standard hobby servos, so one uses a standard servo software library to control them.
You have picked a difficult project as a quad requires many sensors, pretty complex software to work out all the tasks required to keep a quad in the air and controllable. There are several quad specific forum sites that can probably give you much more detailed help then this site, but lets see what others recommend.