I found that I too had this problem. I have tried everything that is suggested here and else were.
What I had to do was maybe a combo of everything suggested here and what I did. Don't know for sure, but I am not going to try to break it again to find out.
After none of these suggestions would get it to work I started digging.
Using Activity Monitor, I could see that when I had the System Preferences open it was taking way to much Physical RAM and CPU, so I selected "System Preferences" from the list of open processes and then clicked the "Inspect" button at the top of the Activity Monitor, under the "Open Files and Ports" tab I could see that the "iTunes Library.itl" file was trying to be accessed. So then I closed the "Inspect" window and clicked the "Sample Process" button. I got A LOT of information, and a lot of error messages refering to iTunes that contained the text "(in iTunesAccess) load address" and a bunch of different numbers that I couldn't tell you what they mean. Searching google for "(in iTunesAccess) load address" didn't give me anything of value, so I figured that since System Preferences kept trying to call the "iTunes Library.itl" file and I was getting a bunch of iTunes errors in sampling the System Preferences process, that the problem might lie in my library.
So to test, I copied about 50 or so songs with album art to the iTunes library of a new user I had just created on my computer. Then I tried to turn on the artwork screen saver, and it worked right out of the gate. So that told me it wasn't a global problem with the computer, just within my profile. Since I have had the same library for about 10 years though hardware, OS, and iTunes upgrades, I thought, maybe if there was some way I could rebuild my Library file it might be happier, and since I don't want to loose all my play lists and what ever historical data is kept in the library file, rebuilding seemed like the best option.
So here is what I found on how to rebuild the Library file, it is pretty easy, but scary to. I have a Time Machine back up of my computer so I felt better knowing that I could just revert back. Before you try this I would make back up copies of "iTunes Library.itl" and "iTunes Music Library.xml" if you aren't backing up your computer already.
http://support.apple.com/kb/HT1451
Then if you have problems with your PodCasts after.
http://support.apple.com/kb/HT2562
After all that everything worked like it was suppose to. Happy day and all the peasants rejoiced.
I really wish Apple would address the issue though, I know it's a small detail, but that attention to small details is sometimes what makes Apple so effing cool. Seems like they are starting to let more fall through the cracks.