yep i had this issue with vodafone myself last year, had 2 contracts with them (mum had one i had other) mums contract (on a samsung f490v) had a addon for internet that was £7.50 a mont hand gave 120mb which was fine, my contract on my htc tytn2 had 'unlimited' internet which i was never ever informed was 500mb so was more than slightly surprised to recieve a phone call one day saying that i was very close to my limit and if i continued to use it vodafone were going to TRIPLE my bill. - i had been on holiday the week previous and was using my phone as modem so yes had used a fair bit of data but i was never informed of the limit, suffice to say i ended up selling both the contract phones to cex and then using the money to pay off the 2 contracts as i dont agree with vodafones business ethics and breech of trading descriptions.

EDIT: i have a vodafone contract currently but i only bought it to get free hdtv (through a carphone warehouse subsiduary company) and its only a samsung tocco lite anyway so no internet on it and i STILL struggle to get a signal where i live, i thought voda were meant to be one of the best networks in uk - thats utter bulls***. t-mobile are good on n900 quick and with webnwalk the fair usage is 3gb (least i think it is ve blown through upwards of 6gb before withotu issue(im still on same tariff i used with my g1 which has web'n'walk plus as part of tariff)