After 5 Lion upgrades, I ran across 2 machines that would not show any Finder windows, I could not see any open folders or the hard drives. They showed up under the windows list, under the Finder icon, and as large thumbnails, after a Show all Windows. But would not show up as a window despite selecting them, or ‘Bring all to Front.’
Other user accounts on the same machines worked, just not the main upgraded accounts. Rebooting did not help.
The solution for me was to Option-Click on the Finder Icon and Relauch it.
[crarko adds: This sounds a lot like corruption of the user’s Finder preferences (~/Library/Preferences/com.apple.finder.plist) or cache (~/Library/Caches/com.apple.finder). If you have this problem and the simple fix of the hint doesn’t solve it you may want to move these to the Trash, logout/login and let Finder create a fresh set. At times in the past, iCal windows were also susceptible to this problem and trashing the iCal prefs and caches was the fix.]