We're having this problem with a Mac Pro running OSX 10.8.4 and 6 monitors with dual graphic cards+Triplehead. The ⌘Tab window does not show and it doesn't switch applications.
I temporarily solved this using this guide by removing com.apple.finder.plist from Library/Preferences, but only for a minute before it reverted to not working again.
Any idea how to permanently solve this issue?