I was trying to solve this problem, too, so I wrote a script that (a) extracts the album artwork from iTunes (placing it in a "album artwork" subdirectory in your Windows temp directory), and then (b) resets the artwork for the selected tracks (and in the process, incorporates the artwork into the mp3 files).
You may download it by right clicking on this link,
http://www.robertmryan.com/tech/ResetArtwork.vbs , and selecting to save the file to your local machine. Then, in iTunes, select the tracks for which you want to integrate the artwork into the mp3 file, and then run this program. This VB script is just a text file, so feel free to open up the script in your favorite text editor if you want to see what it's doing.
This works for me, but iTunes scripting is a little flaky, so I don't make any promises. I used it successfully on Windows 7 and iTunes 9.0 and 9.1. I'd suggest trying it with just one or two tracks first and make sure it works, before you run it on all of your tracks. It's also a bit slow: It took about an hour or so with 10,000 tracks on my reasonably fast machine.
I hope this helps. Let me know if you have any questions.