differences in how the Duemilanove & NG use power?

i had a NG that I was using to control stepper motors that needed 24V of power. I had an accident with the NG and fried it, and the Duemilanove I got to replace it doesn't seem to work the same way. Everything works as expected when it's plugged into USB, of course, I can't power the motors with that. When I plug in the 24V power supply, the Duemilanove's PWR light turns on, but it doesn't do anything. Is there a difference in how the NG and the Duemilanove handle external power? (Am I not able to power these 24V motors with the new Duemilanove?)

weird. if i have both external power and USB plugged in, it works fine. unplugging the USB causes it to act up. (lights flashing when they shouldn't, the arduino acting like certain buttons are pressed when they aren't)