I'm glad what I worked out seems to be true for someone else.
I've now managed to get artwork for every single album onto my iPhone (140 albums, 1,800 songs, so it's not a massive collection).
I think the problem with non-perfect wireless connections can actually strike at either end, i.e. when your iOS device pulls the content from the Cloud, or when your Mac or PC first pushes the content into the Cloud.
Either one can can result in a missing album cover. (Which may be part of why it is so confusing.)
Once you've switched iTunes Match off and on a few times on your iOS device (and you have to wait at least 10 mins or so after switching back on, to see which album covers really come through), then you start to get an idea of which album covers are never going to come through, however many times you try.
For those permanently missing covers, you have to drag the album art out of one of the tracks, then re-apply it to the whole album, in iTunes on the source computer (as per lots of other posts about this issue). However, as far as I can see, you *don't* need to select "Update iTunes Match" after doing this, and it doesn't really help to do so. iTunes uploads the changes anyway, in its own good time, and for me selecting Update Match whilst this is happening causes the Update Match to fail anyway. You can tell that iTunes has finished uploading the changes automatically, once the little stripey cloud icon has changed back to a plain cloud, and once iTunes stops warning you that your data isn't in the cloud yet, if you click CTRL+I again to edit info on one of the changed tracks.
Finally, you can now switch Match off and on (again!) on the iOS device, praying and hoping that you have a *very* good internet connection.
Hopefully, if you do, it will get across all the albums, with all the artwork. (That said, I still have a few albums showing old artwork for the small image, even though they have the latest artwork which I uploaded, in cover flow for instance!! But it's much closer to correct.)
Based on what's happened for me, here, this is obviously pretty sucky, and also anyone with a really large music collection is going to struggle to ever get all their album artwork to appear on their iPhone or iPad.
(I have iTunes 11.0.2.26 on a PC, also syncing to another computer (a Mac) and to an iPhone 4, iOS 5.1.1.)