What your describing is a iPhone without a phone dialer. The 3G connectivity would still be provided via the radio that is what your considering a phone. Just like when your using a cellular 3G modem (like I'm using right now) I have a dialer app that can use it as a phone. There is no separating the 3G without the radio.

I do get what your saying about battery life though that's why I keep my Blackberry as a backup, it only needs to be charged once a week.