I think this sums it up right here:
"In contrast, the EVDO technology on which Sprint and Verizon base their 3G networks supports data only; voice uses older CDMA networks, which (in theory at least) use less power."
So, if Apple could add a firmware update to the iPhone in the future which says "Use EDGE for Voice", I think that might actually significantly improve battery life. Basically when you make a call, the phone could turn off the 3G functionality, and then automaticaly turn it back on after the call.
The reason WHY calls sound so clear on the iphone is because we are on 3G. The older EDGE and CDMA networks sound very compressed on the audio, but when talking to anyone on 3G, it sounds like you're using a landline which is just awesome (but sacrifices battery).