Just a hunch, but it may be because iPhone users are under a specific iPhone data plan and not just a generic smartphone data plan like for android phones. I will be switching from an iPhone 4 too, but I'll probably just go into the store so that they can give me a new sim card.
yea, it is strange. Everything was working fine before the 2.2.3 upgrade, as a I was able to register over 3g. The perform the ota upgrade i switched of to wifi. I tried a reboot, maybe i need to re seat the card.