Many of the Mobile companies are enforcing their "Unlimited" bolt on for example Vodafone from OCt 1st whereas before most people never got charged if they went over their in small print limit.
Its amazing how much data everyday sites in there full desktop version can use. On my Nokia N900 it has a browser which views full pc like webpages and runs flash and a browse on that on 3g for hour or so will consume a huge amount of data compared to the slimmed down "Iphone version" of facebook. A quick browse on my pc of BT sites and whatnot took just under 100mbytes!!!