I'm not sure that you can do that.
If you call your provider, you can have them turn off any data signal, but to buy a new 3G phone, you have to commit to 2 years of the service.
The only possibility that I'm aware of would involve a used phone, and they've been out for such a short time... Good luck with that.
Also, since you can't currently unlock a 3G (yet), I'm not sure that it is even doable then.
Just suck up, and buy the plan... or don't. :shrug: I think those are probably your only options.