Having the same issue with the same device. I wound up having to downgrade to 6.01 (via recovery mode), just to keep the device running. Unfortunately, this means I can't restore my settings from iCloud, until 6.1b5 works.
I had no problem updating my 4th gen iPod. But no matter what I try, I can't get the 5th gen iPod .ipsw (I made sure I'm using the right file for the device) to install without this error.
Wish I had a solution for you. I suspect it may mean Apple posting a new version of the file, for those of us who can't install it.