Some US websites offer computer code “cracks” to allow the phones to work with any service provider.
Apple has signed an exclusive deal with AT&T in the US and O2 in Britain to operate the phones.
Several gadget enthusiast web sites, including Gizmodo and Engadget, as well as online postings from hacker communities now say that, depending on which unlocking program was used, certain modified phones no longer worked after they installed the software update.
Some sites also reported that iPhones equipped with unofficial, third-party applications were disabled after the update.
Other hackers said Apple’s warning was “a scare tactic.”
Company officials insisted they were “not proactively” trying to make hacked iPhones useless.
The iPhone officially goes on sale in Europe in November but will not be available on the Irish market until after Christmas.