As stated the Evo's ONLY* voice calling capability is over the CDMA2000 network. While in a call you will not be able to access the 1xEVDO data network.
The outstanding question is if Sprint and HTC will account for the use of the WiMAX radio while in a CDMA2000 voice session. There are a lot of issues that have to be understood, accounted for, tested, and re-tested. While it might seem easy those who understand even a little bit about how the systems play together know it is not.
*Of course someone could write a VIOP app for the Evo but that is a totally different topic and one for which we would need a totally new thread and lots and lots of time!