Where can I get a prepaid SIM card for an AT&T Wireless GSM phone? The
answer may be obivious - such as AT&T, but I want to know exactly
where to go - web or store, etc. What are my options? What is the
cost? Note: The phone is not "unlocked" it was purchased and used on
the AT&T network with an AT&T SIM card that I no longer have (it was
on contract).

Mark

AT&T Wireless sells prepaid SIM cards under the brand name "GoPhone":

http://www.attwireless.com/personal/gophone/index.jhtml

If you already own a GSM handset (either AT&T or unlocked), you do not
need to purchased any of the advertised phones.  A SIM will cost about
$25.

Rates are similar to a postpaid plan, with no contract required.

http://www.attwireless.com/gophone/rates.html