If one removes the battery on a car for over 24 hours does the computer go back to the default software version that the car came with and have to be updated again? Thanks.

I think it retains whatever was in there when the power was removed from it. The only thing it has to do is relearn some of the adaptive fuel strategies, idle speed settings, etc. So if you loaded a new tune in there it will still be there when you power back up minus the fuel trims etc.

Removing power from the PCM will clear the KAM (Keep Alive Memory). Which is where the adaptive learned settings are stored (as Don noted) and Diagnostic Trouble Codes.