There's a non-free piece of palmos software that allows you to use a
Treo 300 or 600 as a wireless modem connected to your computer via
USB, and the web page mentions Mac OS X, Linux, and Windows. I've
tried it with Mac OS X and it works. Might be worth a look, as the
Treo 300 is already a really nice phone and the Treo 600 is the
updated version, and they both work with Sprint's network: 
http://www.notifymail.com/palm/wmodem/

Hope this helps,
Zach Fine
zach @zachfine.com