I'm certain that bluetooth tethering takes less power than wifi. Go here
http://forum.ppcgeeks.com/showthread.php?t=78019 and download the stealth mode Internet sharing. Once downloaded, open up the new "Internet Sharing" program and select bluetooth. That will start the transmitting.
From there you are on your own. I know it works over bluetooth with a mac but I tried to set it up with a friend's mac and no go. Interface for bluetooth was too confusing. I'm totally not a mac hater to begin with either (SARCASM!).