I guess that depends on their reason for the cost increase. If they claim the cost increase is for 3G service, then you have a valid point. If they simply claim they are increasing the cost for all new iPhone service, then you're probably out of luck. Once the new iPhone hits, I'd try testing them. Call and tell them you want to to start new service with a first gen iPhone. If they say the data part of the plan will be $20, then you have them.
To be honest, I don't know why you would buy the 3G iPhone if you don't have 3G service available. The real greatness of the iPhone lies in the software, which you'll get on current iPhones for free. I'm not even sure if GPS will work without 3G. It doesn't seem right in my head, but that's what Jobs implied in the keynote. (If that is the case, I'll be upset, because shouldn't GPS devices work anywhere in the world??)
If you don't have an iPhone and want to get one for a non-3G area, my advice would be to wait for the 3G phone to hit, then pick up a slightly used one on ebay. You'll be able to get a 16GB version that practically new for much much less than the new model. And in terms of functionality, it'll be able to do everything a new one could do in your area. Anyway, that's what I would do.