So, I've looked around, and I can't seem to find a place where it ANSWERS the problems about using internal interrupts to wake up the Arduino while it's asleep. If I just wanted my Arduino to go to sleep for five minutes and then wake up again, how would I go about doing this? Any suggestions would be greatly appreciated!
You can't - full sleep disables all clocks, so there's no way of counting 5 minutes, without some external interrupt.
I think the best you can manage is to use the watchdog as a source of interrupts, set it for the longest timeout (about 8 seconds) and then count the interrupts every timeout until your target time has elapsed.
The watchdog clock is independent, but pretty inaccurate.
There was a thread in development a few months ago.
[edit]Here ya go: http://www.arduino.cc/cgi-bin/yabb2/YaBB.pl?num=1263247150/15[/edit]