If you cancel within 72 hours you don't pay for anything but the data you've used.
Note that while this is definately the policy, it took me 3 calls to get them to correctly credit everything. If you stay on top of them though, they'll get it done.
Here's the full story if anyone is interested:
I called to cancel my data within the 72 hour period and was told that it was no problem and I'd recieve a bill for the usage, about 15 days later I get my first bill from Verizon with $60.61 in charges which included the first month, plust the acrivation fee, plus usage and tax. The usage amounted to about .65 cents. I called them when I got it and they argued about the activation fee but eventually said they'd credit it and send me a new bill and I should throw out the one I had. Well, a month later I get a new bill for... $60.61 threatening late fees. They obviously canceled the service because there was no new monthly service fee. I called and got someone good who appologized for the error and credited the entire bill, and told me I now have a $0 balance. Hopefully I won't get another bill next month.