If you have a T-Mobile Dash (or some other Windows Mobile Smartphone) and a Mac, take a look at this article and video demo by Brian Jepson…
HOWTO: Tether Your Mac and T-Mobile Dash with Parallels
He explains how a Dash can be used as a wireless modem for an Intel based Mac with Parallels for Mac (virtualization tool) running Microsoft Windows XP as a Guest OS.