Vonage over cellphones?

Sure, we wouldn't mind having a Vonage WiFi phone that we could use to make cheap phone calls from any hotspot, but what we really want is for them to come out with a softphone (a software version of the service) that'll run on our Treo 600. Sprint's PCS Vision network is probably just fast enough to be able to make Voice over IP calls, and we'd be able to use Vonage's unlimited long-distance calling service. That way we wouldn't have to use any of our minutes and could avoid having to pay Sprint for anything except for data (we have a $15 unlimited flat-rate plan). Our old friend Om Malik is in India right now, and basically did this, just using his laptop with a wireless cellular card and a headset rather than a PDA to make Vonage calls to the States that cost a fraction of what it could cost him to make a call using the regular phone network (about 25 cents versus 12 dollars). There's no technical reason why you couldn't do the same thing with a cellphone or smartphone.