I don't want to sound dramatic, but I think I found the solution on the artwork missing on iOS device via iTunes Match. It worked for me in three different albums that was missing artwork, so it should work for all. It is a rather long process and manual, but better than nothing. Here are the steps:
1 - Go to your iTunes in your machine (I am using a 64-bit Windows 7). You should already have all your albums with the artwork on them.
2 - Check your iPhone, and pick an album that is missing the artwork from the iPhone.
3 - Go to your PC, select that album and Ctrl+i in your keboard. It will ask you "Are you sure you want to edit information for multiple items?". Hit "Yes".
4 - Multiple Item Information comes up. Stay on "info" tab. Copy/paste the artwork of that album (you can get it from any website, Amazon is a good one) into "Artwork" at bottom right. Click OK.
5 - Run Store/Update iTunes Match in your machine.
6 - Go back to your iPhone, turn off "itunes match" from Settings/Music.
7 - Power off iphone, power it back on.
8 - Turn iTunes Match back on on your iphone.
9 - The artwork for your album that was missing it, will show up in a few minutes, along with everything that was already there.
Good lukck! As I said, it worked for me on 3 separate albums. Have not tried it with multiple albums before the power off, itunes match off (on iphone). Let me know if it worked for you.