Same issue. Started today and has lasted over 12 hours. Tried two seperate networks, a test user account, cleared iTunes plists and caches, changed iTunes/Apple ID password and a bunch of other things to no avail. Contacted Apple Support and the initial two replies had nothing to do with the issue (waiting on a third reply).
Basicly, iTunes can load the store and all links that are NOT associated to my account. However, if iTunes needs to access something related to my account (i.e. sigining in, authorizing a rental, updating Genius, etc.) it becomes stuck in an an endless loop of "Accessing iTunes Store..." Occasionally it will error out with one of two error messages ("iTunes is unavailable at this time." or "There was an error in the iTunes store."). Both error messages state to "Try again later."
Hoping that this is only a one day, temproary error. As of right now, I am unable to even login into my iTunes account on the Mac (I am able to access my account just fine via my iPhone).