Mark: you just need to make sure whatever contract you have has a good data policy. The iphone doesnt really use that much if you arent on GPRS/3G all the time. But if some mong at work has an ordinary Vodafone contract with no data bundle and uses it, then yes it will get very very expensive! You can quite easily talk Vodafone or whoever into giving you nearly free, nearly endless data when it comes to contract renewal
But remember with iPhone coming to T and Voda, dont be suprised if they hit back at non-iPhone tarriff users who have iPhones on their networks, I can assure you they keep a very close eye on these things.