As noted in the press release promoting the agreement, Apple entered into a "multi-year partnership" for Cingular -- later rebranded again as "Wireless from AT&T" -- to exclusively provide cellular voice and data services for the iPhone.
Steve Jobs explained that this was because Cingular was the "best and most popular carrier in the US". No doubt some readers would debate whether or not the company really is the "best".
No official information regarding the precise length of the contract is available, but given that upon its introduction the iPhone required a two year contract with Cingular/AT&T Wireless for service it was a safe bet that the contract would be for atleast two years from June 2006 in the United States. However, CNN reported that the contract was "through 2009" and USA Today reported that it was for "five years".
A January 10, 2007, article in the New York Times also said that if you want to use the iPhone on another carrier like T-Mobile, "you'll be waiting a long time, my friend" and that "the [multi-year] agreement also include[d] future models which would be introduced 'soon.'"
More recently, on April 15, 2009, the WSJ reported:
AT&T's exclusive deal to carry the iPhone in the U.S. expires next year, according to people familiar with the matter, and [AT&T CEO Randall] Stephenson is now in discussions with Apple Inc. to get an extension until 2011.
Additionally, MacRumors spotted an April 16, 2009 interview with Verizon CEO Ivan Seidenberg where he noted "it is more likely that Apple would be willing to work with the carrier under the fourth-generation, or 4G, network" and speculates that since Verizon plans to launch its 4G network in 2010 and AT&T Wireless' contract expires around the same time, this would be a convenient time for Apple to support multiple carriers.
Since that time, rumors of a CDMA-based iPhone to run on Verizon's network persisted, but AT&T's grip on the iPhone remained as tight as ever. More recentspeculation ponders whether or not this exclusivity will end in 2011, but at this point one would be wise to remain skeptical given the track record of earlier speculation [this final speculation, thankfully, was correct].
For those interested in using an iPhone on a network other than AT&T Wireless in the US -- such as Dennis Sellers from the always excellent MacsimumNews -- one can only try to remain optimistic that AT&T's rein eventually will be coming to an end [it did in 2011]. In the meantime, however, one could consider purchasing an "unlocked" model from outside the US and using it on another carrier that supports GSM such as T-Mobile (Sprint and Verizon use CDMA and are not compatible with GSM phones).
