A few people using the new Mac App Store are finding it impossible to log in because the sign-in window is missing. This appears to be caused by an incompatibility with the popular FruitMenu OS add-on ...
OS X users running Unsanity's FruitMenu application may not be able to log in to the Mac App Store. Unsanity is apparently working on a fix for this issue, which may be a simple oversight in FruitMenu ...