I am having a similar problem..
I had my blackberry for 24 months with orange and never once went over my data charges.
After having my HTC Desire for the first month my bill soared to £320 rather than the usual £35 + Vat. I have contacted customer service and they say there is nothing they can do as i have used the data and therefore have to pay for it, which is what i dont understand as I have not done anything different to what i did on my BB previosly for the past 2 years !
Any Help would be much appreciated