While there has been no official announcement from AT&T about a new upgrade option, several bloggers from TUAW have noticed a change in their upgrade status. I personally checked the upgrade status with a few iPhone-toting friends this evening, one of which only upgraded to the 3GS a few months ago, and everyone currently receives the same new upgrade option.
While upgrading early would be great in terms of getting new iPhone, signing a new contract will force many iPhone users into AT&Ts new data plan. Current iPhone customers have the option of keeping their unlimited data plan indefinitely until they choose to sign a new contract. Current unlimited iPhone data plans run $30/mo for 3G customers, and just $20/mo for original iPhone users. New capped data plans are effective for all new contracts starting tomorrow.
You can check your own iPhone upgrade status by visting your account page on the AT&T website, or by dialing “*639#” from your iPhone.