In a story on Reuters AT&T said that there would be penalties for users who do not activate the iPhone in the first 30 days.
The move is understandable as the grey market of unlocked iPhones is gotten pretty big. Now that there is an SDK the whole jail breaking stuff becomes unnecessary as well.
In some countries the iPhone 3G will be available from more than one carrier already. Maybe this will change eventually in the U.S. as well, but most likely not very soon. AT&T is already complaining that the new low price is hurting its numbers.
More iPhone 3G News. See also all about Unlocked Phones.