Along with iPhone 3.0 Beta 5, which appears to hide away MMS functionality for AT&T, Apple also seeded the second pre-release version of iTunes 8.2, which looks like it blocks editing of the iPhone carrier file. In previous beta releases, developers were able to use the carrier file to enable tethering, something built into iPhone 3.0 by Apple but entirely dependent on as-yet-not-given carrier approval.

Did AT&T ask for the new lockdown? Perhaps the MMS stripping as well? Nothing but speculation so far...

(Thanks Andrew for the tip!)