UPDATE: MuscleNerd (from the Dev-Team) points out on Twitter:
How would AT&T send *any* OTA command to an iPhone4 on T-Mobile Rest of the silly “leak” goes downhill from there
Looks like a false alarm.
How it works is every 7 to 14 days We (Apple) and AT&T send a message to your iPhone that gathers some data from your iPhone without you knowing and we see what is running, baseband and firmware.
Now you can try to work your way around it until you get the second mandatory message again “You must plug iPhone into iTunes and update” If you don’t do this you will then receive no Signal from the network anymore and your iPhone will potentially lock up with no signs of use until you update your iPhone via iTunes.