UPDATE: This issue appears to be fixed as of iTunes 10.4.
This little window keeps popping up whenever I access iTunes:
It appears to be a recent development as of iTunes 10.2.x (I'm currently using "10.2.1 (1)" according to the About iTunes dialog). It shows up after launching iTunes (even if I'm not viewing the "iTunes Store" part of the iTunes Experience™). Afterwards, it can periodically pop up without warning.
Here's a short video demonstrating just how annoying this window can be. I'm signed out here, but the same behavior occurs when signed in. Just trying to search the store for an artist or album will cause the window to show up after each individual keypress:
Some suggest that it's because my computer is not authorized via the Store → Authorize This Computer menu command (or if your computer already is authorized, de-authorize it, then re-authorize). Sadly, my work laptop is not authorized and cannot be. I've hit my 5 computer limit at home and can't authorize my work laptop. Fortunately, since all of my music is not protected, it plays without needing authorization.
So, I'm stuck with it for the time being. It's pretty annoying, especially since when the window pops up, it stops me from using iTunes at all. Surely, whatever it is doing can be done behind-the-scenes in a way that doesn't disrupt use of iTunes itself.
Come on Apple, let's fix that.
Apr. 18, 2011: Still affects 10.2.2 (12).