The short answer is don't bother. I think this idea has occurred to every apache geek at one point or another. It's been investigated and tried and failed over an over again. Out of the box, Windows Mobile cannot serve as a router or bridge between the cellular modem and Wi-Fi connection. With the newer ROMs, it can do this via bluetooth however.
There is a crude hack out there that takes a long time to set up, is very flaky, and only allows port 80 to get through. If you're still interested, here's the link:
http://www.pocketpcmag.com/blogs/ind...&c=1&tb=1&pb=1