DannyBres is correct, the unlock is ONLY available to those with 3Gs that have a 2.11.07 baseband or earlier, it is possible to update to 2.2 using custom firmware [via PwnageTool] as long as you don't update the baseband. Those who have used iTunes to update will not be able to use the 3G unlock as they will have updated their 3G's baseband in the process
Despite having a 1.45.00 baseband I'm sticking with 2.1 for the time being; when I updated my 2G from 2.1 to 2.2 it caused glitch on the iPhone which meant that Cydia would not run, though I did get it working eventually using QuickPwn, but seeing as Cydia is pretty central to the whole yellowsn0w unlock it's a chance that I'm not prepared to take.