Along with iPhone 3.1.2, AT&T users received a 5.6 carrier file update, but the reason(s) for it weren't clear. MMS was already enabled, and tethering was nowhere in site, so what was it for? Well, iPhone Savior sat on the phone with Apple support long enough to find out! Turns out, it fixed an MMS bug:

When sending photos or videos in an MMS message, the progress bar would stall at about 90% then result in an error ending with a (!) red exclamation point next to the MMS message. The associated message failed to be properly sent, though standard text messages worked as normal. The 5.6 patch addressed that particular MMS issue.

Anyone out these have that problem prior to Carrier File 5.6? And if so, did it fix it for you? Let us know!

[iPhone Savior via Gizmodo]