Based on news from Engadget, Business 2.0, and other sources, "unlocked" versions of the Apple (NASDAQ: AAPL) iPhone have begun to go on sale. This means that the handset can work on networks other than AT&T (NYSE: T) Wireless.

The phones will not be available directly to consumers. A company called iPhoneSimFree will offer the software to modify the phone to resellers who already market the iPhone. The handsets can be used on some networks overseas and the T-Mobile network in the US.

For AT&T, the new software is clearly bad news. The phone company is likely to have real problems with losing customers from its network due to their new found ability to change the handset over to other networks. AT&T has spent a huge sum of money marketing the iPhone and it was hoping to switch a number of customers from competing cell providers. That advantage could now begin to erode.

From Apple's side, a report from Piper Jaffray estimates that Apple is receiving $3 a month from AT&T for each iPhone user, and an additional $8 a month for new subscribers to AT&T's network lured by the iPhone. Despite losing some of the cut it gets from AT&T's calling plan revenue, Apple may not hurt badly over time.

Overseas, Apple is also in the process of signing deals with with T-Mobile of Germany, Orange of France and O2 in the UK, according to the FT. Under these contracts, Apple will get 10% of the fees charged for phone calls and data. Unlocked phones would certainly threaten some of that revenue.

Douglas A. McIntyre is a partner at 24/7 Wall St.