It's all about "intent". Obviously modifying the baseband chip firmware in the new 1.1.1 update was NOT "necessary" and Apple only did it to re-lock all those unlocked phones. Thus that intent is considered "malicious" and a pretty good civil class-action can be made about it since the DMCA specifically exempts anyone from being sued by a cell phone company for "unlocking" their own phone. IN a class action suit Apple would have to provide PROOF that there was a specific bug in the baseband firmware that was bad enough to necessitate such a forced change.
If Apple cannot provide proof then the court system will interpret that as malicious intent and an actual violation of the current DMCA exemption and a curtailing of consumer rights. That could set one hell of a precedent and you can be damn sure that every single cell phone provider and manufacturer on the planet would be watching such a case very closely.
Again, Apple doesn't have to support any user-made modifications to the iPhone but they also CANNOT maliciously "brick" users' modified phones just to drag them back to AT&T or to force them to buy another iPhone.
Magnussen-Moss is being touted as having something to do with this even though that act was originally created to deal with the automotive sales market.
Again, it all goes back to this ONE question:
"What was Apple's purpose in having their update reflash the baseband chip?"
If they cannot come up with a good enough reason then Apple is legally sunk as far as the iPhone is concerned.

However, current US law does NOT specifically state that cell phone providers MUST allow the cell phone purchasers to be able to unlock their phones for use on different networks. There is legislation currently being drawn up to address the issue, but it's not been sent down to the floor for a vote, if it ever makes it out of committee. But current US law DOES allow for user's to unlock their own phones themselves without fear of reprisal from the phone manufacturer or cell service provider. Again, this current update could be interpreted as a malicious reprisal by Apple towards those who unlocked their phones to run on GSM networks other than AT&T-owned and operated networks.

I care not one way or the other really since I don't own one of those crippled-from-the-start iPhones but when one looks at the whole situation from a legal standpoint, Apple could be in big trouble if the can't answer the baseband chip reprogramming issue.
Of course, Apple could just come back and say that it was needed in order for the phone to support more GSM networks world-wide, but that's shaky ground and actual code would need to be reviewed by a court-appointed neutral third-party or special master to verify that claim.