The UK cell carrier O2 will not enforce a "fair usage" policy on iPhone data plans, the British newspaper The Telegraph reports [1]. Although I could never nail down precisely what "fair usage" meant in this context, the Telegraph writes that O2 was planning a 200 MB per month limit (see "iPhone Launch Set for UK and Germany, with Murky Data Plan [2]," 2007-09-20). That limit has now been lifted, and the contract terms on the O2 Web site changed.
The O2 head said, "Customers find 'unlimited with limits' confusing," which shouldn't be a surprise. Verizon Wireless just settled [3] with the Attorney General of New York over the firm's advertising its cell data service as unlimited, while limiting customers to 5 GB per month and restricting permitted activities in certain ways.
[1]: http://www.telegraph.co.uk/money/main.jhtml?xml=/money/2007/11/03/cniphone103.xml
[2]: http://db.tidbits.com/article/9189
[3]: http://www.oag.state.ny.us/press/2007/oct/oct23a_07.html