I've just had the same problem with Orange. I'm still waiting for a final answer but the operator agreed with what I said. Basically I had a montly bill that came to £1580. Obviously this seemed excessive but it was basically all data download. They had a record of 1003 MB downloaded in the month, I have 500MB included in the contract, so it was 503MB over my allowance. It turned out they had charged me at the rate they used pre 2009, which was £3/GB and they hadn't put the daily cap onto my account. The daily cap is £1 and there are no further charges, so even if you went over everyday in a month it would be no more than an extra £31. Might be worth querying that with them.
Anyway the link to the website with the relevant information is here:
all that said I'm still to receive official confirmation from the billing team that they are going to reduce my bill to what it should be!
Let me know if anyone raises this and how you get on.