I'm having the same problem as the original poster. I'm logged in to my account. I
know I have an existing balance, because I just bought a song. But since the upgrade to 10.0.1.22, all that shows in the top right corner is the email address connected to my account, whereas before it used to show my iTunes balance. That balance doesn't seem to be discoverable anywhere anymore.
The pic is what I'm seeing since the upgrade:
http://img.photobucket.com/albums/v721/onyerbike/iTunesmissingbalance.jpg