I have JUST discovered something that worked for me. This issue has driven me CRAZY for two days on my iMac while it's not a problem on my MacBookPro. What I did on the iMac that SEEMS to have resolved it so far is this:
System Preference > Mission Control. I unchecked everything except Show Dashboard as a space. I restarted and now I can see my Finder windows. One other change that I made (to be thorough since I'm not sure WHICH one made the difference is I opened the Hot Corners dialog and assigned SOMETHING to every corner. Make SURE to reboot after making the change. I closed System Preferences and tried Finder and still got nothing. Decided to reboot on a hunch and it worked!
When I installed Lion I had something assigned to every corner but then with LauchPad, I had decided I didn't need something and unassigned that corner. If I'm not mistaken, that was about the time that the Finder issue started. I know I wasn't haven't an issue with Finder before that because I opened it and did an Air Drop file transfer with my MacBook Pro to check out that feature. I have no idea if this resolves my problem for the long haul but at least I can see my Finder window right now - wait.... Yep, it's still working so far.