Engadget digs up some 2008 court documents that confirm that Apple and AT&T's original exclusivity agreement did, in fact, extend into 2012. From a court order (PDF):
The 5 year timeframe was originally revealed by USA Today back in 2007. A 5 year contract would have maintained AT&T iPhone exclusivity into 2012.The Agreement, which lasts until 2012, provides that iPhone purchasers who want voice and data services must sign a two-year service contract with ATTM. (Complaint P30.) Although the Agreement itself is not public, some of its provisions have been revealed in the press.
However, at this point, it's not entirely clear if the original contract still stands. The Wall Street Journal reported in 2009 that AT&T's iPhone exclusivity is due to expire this year (2010). Apple and AT&T may have undergone contract revisions and negotiations since that time. Most industry experts seem to believe that AT&T exclusivity comes to an end this year with several whispers that Apple will bring the iPhone to Verizon's network in 2010.
Article Link: AT&T iPhone Exclusivity Originally Scheduled Through 2012