Russell:
The external interrupt scheme to re-wake a SLEEP (Power Down) mode on a microcontroller is indeed a good way to manage powerdown.

There are some 8051/8052 derivatives that will not re-wake from a SLEEP (Power Down) mode via an external interrupt. For these parts it may be necessary to arrange circuitry to interface with the RESET pin. Another scheme is also possible on some derivatives as well. For example on a Cygnal part I have worked with there is a way to utilize the watch dog timer as a repeatitive re-wake trigger. It is possible to create a very low duty cycle scheme to kick start the processor out of Power Down via the watchdog reset. Then the processor software can quickly interrogate the switches and go back into power down if no switches are active.

Many ways to scratch the dog's back ... :-)
Michael Karas