I just got my Treo 650 to act as a modem with Linux. It was relatively easy once I found the right information on how to configure pppd.
First I installed USB Modem on my Treo. Pretty straightforward. I got the trial, it's allows for 30 connects.
I read the docs for USB Modem and it didn't seem to help. I guess Sprint doesn't have things set up the way that USB Modem assumes, but it is nice that they attempted to help us penguins out with pppd scripts.
When I tried to connect with the pppd scripts that came with USB Modem, pppd keep reporting that my authentication was being rejected (I put in my sprint vision username and password). At first a assumed I had my password wrong, so I gave up.
I eventually found this page: How to Setup Linux to use Sprint PCS wireless web access via data cable with Samsung A460. Although the page described how to connect your computer to a Samsung A460, I hoped that it would work for my computer. It seemed too simple, use the username, "web" and the password "sprint". After copying the scripts from the website, I fired up pppd and sure enough, it worked. Yay!
Unfortunately I have a Treo 650p, so I can't get any EVDO goodness like I would with a 700p. Whenever I upgrade to a 750p, or a 2050p (the model number when I save up enough to buy another) I'll be able to cancel my broadband account. However, with the 650p, I get speeds roughly around 10kbs a second (Welcome to 1995).
Gven that it's very slow and most things I need to do on the Internet, I can do directly on the phone (Mail, Web browsing, etc), I don't see myself using this all that much. Since the majority of sites nowadays assume you're on a broadband connection, using the phone's specialized web browser makes the 10 kbit/s connection less noticeable.
Here are the settings to remember if you're trying to do this in Windows or OSX:
username: web
password: sprint
authentication mode: CHAP
