Just to add my experience to this thread...
I recently changed cell phones and went from the Sony Ericsson P800 to the PalmOne (formerly Handspring) Treo 600 PDA/Phone (a long story as to why). This Treo 600 was supplied by AT&T Wireless (GSM band radios).
I'd been with AT&T for many years but I wasn't enamored with my data plan (I had the 8MB per month plan for $20). Recently, I network unlocked the phone and moved my number and service over to T-Mobile. I did this to get T-Mobile's unlimited data plan for the same $20 per month I was already paying. And while this may be specific to my area (Chicago suburbs), my coverage is much better.
That all said, what I did next is the point of my post. Using the sync cable that came with the phone and using a Palm OS application called WirelessModem (
www.notifymail.com/palm), I successfully connected my Mac Powerbook to the Internet, using the Treo 600 as a wireless modem. So far, I've tested email using a web client (and mailblocks.com) and MSN Messenger (for Mac). Both worked fine.
I am using the WirelessModem application as a demo at this time. When we're out on our maiden voyage with the TM 3023 this weekend in Michigan, I'll give the Phone/Laptop setup a real field test. If it goes well, I will register the application. It's not cheap ($37.50), but if it works well, for a one-time investment of $37.50, I can really leverage that unlimited data plan.
Take care all,
Jim