The Duemilanove is not prepared to run with external 5 Volt.
Really? Is that documented/explained anywhere? I've been wondering how the new power circuit interacted with +5V via the power connector
We (or I, anyway) was talking about feeding a regulated 5V into the "5V" pins on the bottom "power" shield connector
You can probably rig up something that feeds in +5V via the USB connector (disconnecting the USB power and using a common ground...)
The Duemillanove regulator drops only about 1V (Vin to +5V), but the switchover cicuit senses on Vin/2 versus 3.3V from USB. So as long as Vin is less than 6.6V, you should not connect the board to USB.
wire the voltage to the two USB power wires