From a press release about some new VOIP software for the Motorola Q:
Lasater explains the value proposition for the consumer, "The cost conscious consumer can now pay a flat fee of $50 per month to Verizon for unlimited EVDO access, or $25 per month to Sprint and use PeerMe from their cell phone to talk to anyone anywhere in the world for an unlimited amount of time. Unlimited data plans of high speed broadband wireless combined with Windows Mobile 5 has completely changed the landscape for the consumer."
You could do this... except it would be violating your EVDO Terms of Service. I can't believe they would put that in a press release, when the following text is in bold in the ToS.
Unlimited NationalAccess/BroadbandAccess services cannot be used (1) for uploading, downloading or streaming of movies, music or games, (2) with server devices or with host computer applications, including, but not limited to, Web camera posts or broadcasts, automatic data feeds, Voice over IP (VoIP), automated machine-to-machine connections, or peer-to-peer (P2P) file sharing, or (3) as a substitute or backup for private lines or dedicated data connections.
(And it costs at least $59, not $50, and that's only if you have a voice plan as well, which diminishes the savings of having VOIP.)
Moral of the story is, don't use EVDO.