I've never really known - why is it that if you have a freshly restored phone, or even a new phone out of box and you activate and jailbreak and put in an ATT sim it still won't work for calls or edge until you unlock the baseband?

I was always under the impression that unlock meant for use with "other" carriers than ATT, but this doesnt seem to be the case. The iTunes activation must activate the baseband as well. I even tried (after A restore) putting my legit iTunes activated sim in an activated and jailbroken phone and it wouldn't work. That was even an iTunes sim in the same phone with the same imei!!! I'm working fine being legit ATT activated and jailbroken, but am curious about this because this could be really important with the rumored in store only activation with the iPhone 3g because I am using gophone.

Can anyone explain this ?