I got it working. What many of us seem to be missing is after setting up the APN, you have to restart the phone at least once so the sim is properly provisioned for data access. These are the correct settings for AT&T (gophone or postpaid):
password: CINGULAR1 (case sensitive)
MMS Proxy: wireless.cingular.com
MMS Port: 80
APN Type: none
In addition, YES, a hard reset is necessary - the Tattoo only likes to take your google account information on that initial boot. I had to reset twice because the first time around, I skipped the account setup and could not get it working even with the 3G mobile network enabled. During the initial boot, I did not have time to setup the wifi so cannot confirm if you need to use the data line portion of your sim to initiate contact Android market to get your google account setup.
After getting things setup the first time around, it works flawlessly on wifi with my blocked GPRS sim card.
$10 for the sim + 1MB of data to get my access to android market - man. Anyway, if anyone can confirm that the hard reset trick with mobile network off and wifi will still allow you to setup your google account for android market, that would be great!