This is unexpected good news for current AT&T iPhone customers. Our used iPhones will become more valuable, particularly in the overseas market. It will be even simpler to use cost-effective services like H2O Wireless. The value and price of a used iPhone should increase, though it never fell as much as I'd expected (perhaps because the overseas device demand remained high despite AT&T's carrier-lock).
It's bad news for the unlocking industry, though they will still have work to do. It will also decrease Apple's sales of its $700+ unlocked devices.
It's mixed news for AT&T. More customers will do as we have, handing old iPhones off to our kids (not to bring to high school though!) with H20 Wireless SIMs rather than paying AT&T's high family plan texting and data fees. That will reduce AT&T revenue, but, on the other hand, AT&T wasn't earning any friends with their unusually restrictive policies. Macintouch reader reports have been aflame with posts by irate customers newly discovering that the phone they'd purchased for by contract and fee still belonged to AT&T. I'd sent my own letters off to a senator and the FCC. I've also wondered if a creative lawyer might decide that post-contract carrier locking was a form of theft.
The policy wasn't earning Apple any friends either, since AT&T routinely shifted blame to Apple. (Since Verizon and Sprint have had much less restrictive policies few believed AT&T.)
So why did AT&T, a carrier desperately seeking fees to offset the end of SMS, cave now? Did the timing have anything to do with Tim Cook's forcing Apple to unlock a single customer's iPhone two weeks ago? Did AT&T jump, or were they pushed?
I think, even though the decision is somewhat in AT&T's interests, that they were pushed. Pushed by Apple, pushed by the FCC, pushed by US Senators, and, recently, pushed by the press. When sites like Macintouch and mainstream blogs start to figure things out, the NYT is not far behind .
I'll try unlocking our 3 older devices in a few weeks and I'll post what I learn on tech.kateva.org.
 Even as Apple enables iMessage on data and FaceTime on WiFi for all iOS devices. Sometimes I wonder if Cook despises AT&T.
 Marginal stories like this one take years to get from geek and specialist blogs to the NYT. Bigger issue stories tend to break everywhere, not least starting with the NYT.
Update 8/31/2012: A bit late, but after 3 unlocks I write a summary post.