I'm waiting for my Treo 300 from Amazon, and I'm wondering the same thing. I know
www.futuredial.com makes SnapDial, which allows you to use a phone as a modem, but the Handspring isn't a supported unit. I've emailed them to see when (if ever) they are going to add support for the 300. If anbody knows a way to hack this thing, please let us know.