For quiete some time it bothers me that i’m not able to update apps for my iPhone / iPod Touch directly in iTunes. I get a notification that some apps have been updated, but when i click on this link, i enter the infinite loop of login to the appStore: i enter my username / password correctly, but get the login window again and again and again …
Today i finally found a solution for that problem here.
Nearly at the end of the thread user “X Rookie” got the right hint for me. The problem wasn’t iTunes or Mac OS X, the problem was NetBarrier! I use it as my personal firewall and like it, but somehow a setting in the data privacy part of the program caused all the trouble and i didn’t set it 
After unchecking the “hide my AppleID when using iTunes MiniStore” checkbox, the problems with the appStore updates are gone 