So far for me it seems to be only affecting iTunes and only affecting updates or purhcases - does not affect iPod or iPhone or my ability to log in and see my account details - just to get updates.
I am having the same issue on a 10.6.8 and a 10.8.0 system - so Mountain Lion is not to blame.
At first I thought my iTunes was damaged by a ssytem reboot that happened about an hour before I woke up (MS database something or other caused it). but with the same error on another sytem that seems to be ruled out.
On my wife's netbook - launching iTunes gave me an error related to Music Match - that asked me to log out and in again. But then the same account unavailable message - sonot unique to Mac verson of iTunes.
So the only common factor left on my end is my internet connection - however, my iPhone and iPad are using that same connection - so seems unlikely a problem on my end - especially if other folks are seeing the same thing.