I tried USBModem over USB on Vista. Works like a champ.

Installed fairly easy too.

Install trial from Treo 700w USB Modem
Plug in USB Cable, let Active Sync do it's thing.
Start USBModem
Choose USB connection and hit Enable
At this time Vista will install a Treo USB Modem driver.
Then goto into Network and Sharing Center on Vista
Setup a new connection (choose Connect through a dial-up connection to the internet)
You should see the Treo Modem there, choose that
You only need a phone number and nothing else (#777)
Then connect and you should be up.

You'll end up with a shortcut in "Manage Network Connections" that you can copy to your desktop for subsequent connections.

Phone does not show a connection is up to internet when connected.
USBModem shows no change in status when connected either oddly enough.
But it's up.

I want to confirm Sprint will really charge for PAN connection before I purchase it.