After hours struggling with this issue I finally found a solution (my setup is iTunes 10.7, OSX 10.8.3):
(But my workaround is only for people who know how to handle iTunes libraries!!! And backup first!!)
Open a new user account on your mac.
Generate a new iTunes library called 'itunes', no other name!
Import some files with embedded album art.
Drag such a file back to desktop and you should see the icon of album art.
Everything is like it should be.
Now copy this new itunes library to your user account of origin.
But first rename your old iTunes library different from 'itunes' e.g. 'itunesorigin'
Open the new iTunes and drag one file to desktop, now miraculosly all audiofiles in all of your itunes libraries should show up album art IN FINDER including preview and quicklook.
But this state is not stable. As soon as you open the old 'itunesorigin' the problem starts up again.
So, the next steps are:
Open the new iTunes and in preferences select the media folder of 'itunesorigin'. OK, say no to 'collate data'. Close iTunes.
Now copy and replace from 'itunesorigin' to the new iTunes the following files:
iTunes Library Extras.itdb
iTunes Library Genius.itdb
iTunes Library.itl
iTunes Library.xml
Open the new iTunes and it should look like the old 'iTunesorigin' with all your playlists etc..
Close again.
Move the media folder from 'iTunesorigin to the new iTunes folder (replace).
Open new iTunes and select in preferences the new location of the media folder.
Now you should have the file iTunes Music Library.xml instead of iTunes Library.xml.
(iTunes Music Library.xml is the default if you have multiple libraries)
Done! For me its stable and I am happy. Your iTunes folder should look like this picture: