Why can't I sign into the Mac App Store after updating to 10.6.6? Ever since I updated to 10.6.6 a week or so ago I've wanted to use the Mac App Store but I can't sign in.  The Sign in link and the sign in menu item neither seem to do anything.  I'm signed into iTunes just fine, I can use my account on my Mac and iPhone.

I've tried erasing the contents of ~/Library/Caches/com.apple.appstore/ but that had no effect.
Got an answer from the apple forum.

Sign out of iTunes.  Log off, log back in.  Then sign into App Store, after that, sign into iTunes.  

Looks like everything works now.