I just made a call while wifi surfing, including watching an uploaded video off this forum, and I had the music app playing as well. By the way, weird hearing 3 audio sources running at the same time on top of each other (gotta love multitasking on this phone).
Evo cannot do data and calls at the same time due to the radio. If you try, you will likely get the call connected and then have it begin to freeze or skip audio. My data just freezes though and does not complete the connection. If you're on 3G data connection, the phone allows you to accept the call, but it stops data, or continue data and send call to VM.
Will have to try while driving some time. I have used GoogleNav and gotten a call before and both the call and continuous Nav took place simultaneously. That was before I updated to Froyo, but I don't see why that would matter. Also seems like there might be a workaround for Nav since it's the one time when data + call truly makes sense, right?
GSM's better than CDMA for data + calls though. Just have to live with ATT or TMo, and it's not worth it.