I've removed the USB connection, so that's a non-issue. I have also removed the power-source-selector, so the only power can come from the dc power jack.
Basically, the mega will have to power:
A gyro
An accelerometer
A compass
ladyada's xbee breakout (well, same circuit anyway)
A small motor driver
3 Servo's (plan to run these from their own power source, as each will run at about 750mA)
4+ Ultrasound sensors
GPS module (ladyada's GPS shield circuit)
Perhaps more...
That's quite the amount of sensors, eh? I want to run everything on that list except for the servos from the 5v line going through the atmega.