This is more of a GSM hint and common knowledge for GSM users, but relevant for iPhone users of this site who may have switched from CDMA carriers. Using standard GSM codes, you can change the numbers for conditional forwarding, such as forward if no answer, forward if busy, forward if out of reach. The default is to forward to voicemail, but there are times when you might want to change that. For instance, to forward calls to your home phone if you don't answer, dial this number (then tap the Call button)...
*61*1234567890#
...where
1234567890 is your 10-digit home phone number. Dial
##61# and tap Call again to cancel that setting. When you do cancel, AT&T inserts the proper voicemail number, so everything will be as it was before you started. I do not know how it affects your minutes or if AT&T charges more for forwarded calls.
Here's a complete list of conditional forwarding commands.
[
robg adds: I haven't tested this one.]