We are hearing numerous reports that AT&T unlimited data users aren’t able to upgrade their unlimited plans when they order the4S. They are automatically changed to a different data tier (in the below case, the $30 data). We aren’t yet sure if this is policy or just glitches in the ordering system. We’ve reached out for official comment and will report back if we hear anything.
Update: It appears the upgrade is just a bit more complicated. You have to remove the “unlimited data for” plan and then they’ll let you add an “unlimited data for ” plan. Yes, unnecessarily complicated and is causing some users to lose their unlimited status.