Yes it does disappear when on a call.

 

I believe that is true. Mine does it and all my other phones did too.

 

Yea, I think CDMA Technology doesn't use 3g for calls, and you can't use 2g and 3g at the same time. Correct me if I'm wrong.

 

Yes I believe this is correct, 3G is used for data only. My Droid has always done this (the 3G logo disappearing and showing bars only) while on calls.

This, of course, means that you can't use data while on a call (pretty much the only advantage AT&T/IPhone has over Verizon/Droid line, although that is basically negated when you compare overall signal quality/coverage anyways).

 

All 3G CDMA phones will lose the display this is also why on the droid if you quickly hang up and then launch google talk it will tell you that you are offline while it grabs a new address.