Any way to turn it on and off without touching the wall wart or incoming line?
The standard Arduino Duemilanove is hard wired from the power connector to the regulator. I can't think of any way to interrupt power without cutting a trace on the pcb.
The seeduino has a switch to select the regulator or USB input. Setting it to USB effectively kills power to the board if USB is disconnected. So it can function as an on/off switch in that case.
As Brian suggests, sleeping may be your only alternative to a complete power down without interrupting the power input.
-transfinite