Does anyone know if it is possible to use a T-Mo SIM card for data downloading (i.e., to surf the Internet)?

The T-Mo rep told me it wouldn't work, and I'd have to go for the monthly data package.

I'm trying to make this as cheap as possible, and if I could save money by paying per minute instead of huge monthly bills I'd much prefer it (I don't use the Web/phone that much to justify the cost, but I still want them available).

Thanks to everyone so far for the help, and thanks to anyone with any additional info.