iTunes prior to 9.0 used a click on the zoom widget to convert to the mini player, leaving people like me who actually like to zoom a window option-clicking it.
iTunes 9.0 finally fixed this. A click on the zoom widget actually zoomed the window!
iTunes 9.0.1 changed it back to the old behavior. Many people were happy, I imagine. But people who wanted to zoom iTunes windows (or, I imagine, valued standard behavior) were left sad.
It turns out you can get the click-to-zoom behavior back:
defaults write com.apple.iTunes zoom-to-window -bool true