http://store.yahoo.com/gomadic-new/nexidi5mobph17.html
this is the easiest way to do it.
its a data cable that goes from your nextel to the sync port on your axim. its only 39.95.
you'll need to have Wireless Web service from nextel, along w/ the ability to use your phone as a dial-up modem from them. The messaging package is $15. This will give you 2-way messaging, full internet on your phone, and the ability to use your phone as a dial-up modem.
This, however, will tie up your phone and use minutes. If you don't have a lot, this may not be for you.
Alternatively, nextel offers a package called Packetsream, which will give you a direct connection to the internet w/o the need to dial-up, w/o the need to use minutes, and w/o the need to tie up your phone. These packages get quite expensive... The unlimited packages if 54.95/month.
W/ the packetstream gold option, you can get a wireless pcmcia card so you don't need to use an actual cable tho... If you have money, that's the way to go.