I'm having a similar problem.
Tried to open iTunes yesterday - said yes to the update question and thought it hadn't worked as the App store said it couldn't access the internet! Ever since then I get an error message telling me that the iTunes store is temporarily unavailable every time I try to access the store.
Checked the version I'm using and it appears to be 11.1 - difficult to tell as the words scroll down too quickly to read properly. My OSX is 10.8.5 if that helps the clever people!!
Apparently I solved my own problem after doing some research. One suggestion I found was to check for software updates. I selected Software Update from the Apple Menu and found out I had 7 items that needed to be updated, including a firmware update. After I completed all that and restarted my MacBook, I was able to access the iTunes Store. I'm assuming something in those updates fixed the problem.