The new tariffs replace the "unlimited" data tariffs offered with the iPhone on O2 in 2007 with a new tiered set, including 500MB, 750MB and 1GB data packages, along with unlimited Wi-Fi time via The Cloud and BT's OpenZone services. However existing users with iPhone contracts will still be able to use "unlimited" amounts of data, O2 said.
Just to clear it up for some people.
http://www.guardian.co.uk/technology/2010/jun/10/o2-iphone-tariffs-unlimited-end