I have the same problem, the artwork is randomly not displaying at all on grid view from my 20,000+ songs collection. For now if you want to do this, which is a pain since there is no preview box at the lower left hand corner to check (even that is a pain), I just do the following:
For example, a song that you know had previously showing artwork, open Get Info, and go to Artwork tab. Your artwork should still be there but not displaying after iTunes 11. Drag the artwork out to your desktop, delete the artwork inside Get Info, and then drag the one on the desktop back. This will make iTunes think there is a new Artwork just added. And you will see it on the grid and it will display correctly once again.
Or better just hope that the next update will correct this problem by itself if you have a large library.
Well I really don't know if iTunes 11 lives up to the hype. It may looks cleaner but the functions are going backwards especially in the MUSIC library. It may look better in iTunes Store or when you plug an iDevice to it. But for the music library, it is definitely many step backwards.