Reverting the buttons to a horizontal layout can easily be done. With iTunes not running, open Terminal and enter the following code:
defaults write com.apple.iTunes full-window -1
To restore the vertical layout, simply use this command:
defaults write com.apple.iTunes full-window -0
[crarko adds: Thank you very much. iTunes 10 has a lot of good points, like better performance, but some of the UI changes are perplexing.]

