Just finished a pair of adaptors for my VEX-Bot using the Arduino.
All parts available at Radio Shack for a little cost, though I did find one item cheaper at another place (a hobby store).
7.2V 3300mAh Ni-MH RC Battery (RS Part# 23-319) (strongest battery I've found, and for the lowest price!)
9.6V RC Battery Repair Kit (RS Part# 23-445, though I found same at a RC Hobby shop for $2.00 less.)
Type-M Female Coaxial Power socket (RS Part#274-1577)
Type-M Male Coaxial Power plug (RS Part #274-1569, 2-Pk)
Simply take the Male end of the RC Battery repair kit, and clip the crimp connectors off (on the Hobby-shop pair, I didn't need to as they were bare wire ends) Insert them through the plastic cover of the male type-M coaxial connector. Either strip (or clip back) to 1/16" of wire on the ends. Solder the Red wire to the center connection, and the black wire to the outer connection. You might need to put some plastic or paper between the contacts to prevent them from shorting. (which is why I chose the 9.6V repair kit, smaller wire, as the 7.2V kit has much larger gauge.) Replace the cover of the connector, making sure nothing shorts, and the connections will fit inside. Using a Volt/Ohm meter, check all connections for continuity or possible short circuits. Connect the 7.2V battery to the mating connector of the new adaptor. Plug the Type-M connector into the power socket of the Arduino. You now have power to your project/sketch!
The VIN pin on the Arduino will now carry 7.2V power, Plenty of power to move servos! (Do Not rely on the +5V output, as the draw could potentially burn out the +5V regulator.)
The separate adaptors: