Well I believe I have found the solution. I like many others was not able to see and use my finder window even though I could "see" it while scrolling between desktops and full screened apps. I tried the solution of clicking on the finder icon in the doc and selecting "options", but I did NOT have this available. I used the temporary fix of: alt, command, esc: and relaunching finder, only to find that after reboot the problem was still there.
So, like Rettogo stated above, go to: System Preferences>Mission Control> and UNcheck "Automatically rearrarange spaces based on most recent use". This was the only change I made, did a reboot, and finder window is now visible and usualble.
I think the problem exists since in SL I had assigned finder to a "space" and after the installation of Lion, those assignments did not fluidly translate. Hope this works for all.