AT&T has moved up the iPhone upgrade eligibility dates for a number of iPhone 3GS users, signaling the imminent arrival of the fourth-generation model. Several readers, along with at least two iLounge editors, report having their AT&T upgrade eligibility status updated over the weekend, changing their dates for eligibility from later in the year to immediate eligibility. This is the second time AT&T has changed some users’ eligibility dates prior to the fourth-generation model’s introduction. The company moved some eligibility dates from November 21 to June 21 last month; it’s unclear whether these same users are now available for immediate upgrades. AT&T customers can instantly check their upgrade eligibility dates by calling *639# from their iPhones. Apple CEO Steve Jobs is expected to officially unveil the fourth-generation iPhone during his WWDC keynote address, which begins at 10:00 a.m. Pacific Time today.