What would I lose from switching from iPhone 4+ e-reader to dumbphone+iPad
It seems to me that you would spend less money overall if you just kept the iPhone and got a wifi-only iPad rather than switching phones and plans (not including that nasty early termination fee on the iPhone contracts). That is what I have now and I really like it.

But, to answer your question specifically, even taking into consideration the iOS 4 release for iPad later this year, you will be missing, at minimum, all of the camera-related functionality of the iPhone, but then there is visual voicemail and easy (and cheap) email. That's a lot to give up all things considered.