One potential use would be for an iPhone user living or traveling outside the United States to access the iPhone's music player and Internet service over Wi-Fi connections without using the phone. Apple has yet to reveal network operator deals in markets outside the United States.
AT&T spokesman Mark Siegel said it was necessary to activate the iPhone on AT&T's network to ensure optimum performance. Using the phone without AT&T's two-year service contract was unauthorized under the phone carrier's exclusive U.S. network service contract with Apple, Siegel added.
"Any other use of the device is not authorized and we can't guarantee the device will perform as intended to. We'll monitor situations like this and if necessary we will take appropriate action," he said. "Our terms and conditions are very clear."
He did not elaborate on potential action AT&T might take.
An Apple spokeswoman was not immediately available to respond to Johansen's claims.
Neither Apple nor AT&T have disclosed sales figures since the iPhone went on sale in the United States on June 29, but some analysts have estimated sales of up to 700,000 units for the costly coveted phone's first weekend on the market.
Johansen became known as "DVD Jon" earlier this decade for helping to reverse engineer the code used to protect DVD movies against piracy, saying he did so in order to play them on his Linux computer.
The Motion Picture Association of America and another trade group charged him with breaking the DVD encryption code but he was acquitted by a district court in Oslo in 2003.
Since then, the computer activist has engaged in a cat-and-mouse game with Apple to bypass copyright controls known as "digital rights management" (DRM) on various Apple products, including QuickTime, iTunes and Apple TV.
He has also taken aim at digital restrictions built into electronics from Microsoft and Sony and vowed to circumvent the security code for next-generation DVDs.
During 2005 and 2006, Johansen relocated to California, but now splits time between San Francisco, where he works for DoubleTwist Ventures, which makes software to unlock copyright-protected music on iTunes, and his native Norway.