Been there, done that. My unexpected bill was over $2000 for one month. Fortunately, its a corporate plan, and I had a multi-million dollar corporation going WTF? to Bell Mobility.
I have been provided with a Blackberry 7130e by the corporation. We picked it because of its ability to be used as a high speed modem, enabling me to support the office while I'm traveling. The account purchased was an unlimited voice and data plan...
You'd think that unlimited data would cover the modem use option.
We were quite surprised. As was he.