I've looked into this issue a lot, and there is no way to legally get around the mandatory data plan. What I ended up doing is upgrading to an Imagio through Verizon and bought a new Motorola W755 on Ebay. When I got the Imagio, I activated it for a minute to accept the terms of the contract, then had the rep credit back the prorated data charges.
Now, I'm using the W755 as a phone and the Imagio as a PDA. I'm still carrying two devices, but it's much lighter than the Treo 755p and Treo Pro combo I had been doing before.