A report in Business Week suggests that the exemption, which is in place for three years, covers computer programs that 'enable wireless telephone handsets to connect to a wireless telephone communication network, when circumvention is accomplished for the sole purpose of lawfully connecting to a wireless telephone communication network'.
Individual users could therefore legally unlock an iPhone until the exemption runs out in November 2009.
The two firms are expected to claim that a statement within the DMCA protects the iPhone from being unlocked because it is a copyrighted work.
The DMCA bill reads: 'No person shall circumvent a technological measure that effectively controls access to a work protected under this title.'
However, users who unlock their iPhone will not escape AT&T network charges completely if they move to another carrier.
The Apple phone, which went on sale on 29 June, was sold with a two-year contract and includes an early termination fee of $175.
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