My wife has an iPhone 4S (iOS 5.1), and is unable to receive picture messages via iMessage. We're in the US on AT&T. Normal text messages go through fine, and if iMessage is turned off, she can receive pictures.

I've tried everything- restoring the phone and setting up as "new phone"; turning off iMessage and MMS, then turning back on. Also tried resetting network settings, all to no avail.

Anyone have any idea what's going on? The phone has never been jailbroken either.