Like many it seems I've been having problems since upgrading to iOS5 and iTunes Match on my iPhone 4 and Windows 7 PC.
I've re added missing album artwork using a combination auto download and manually adding (now selecting all tracks) to each album iTunes/gracenote can't find.
This however did not resolve the issue of missing art work on my iPhone, which is particularly frustrating as this acts as a visual link to the album and artist (my instant recognition of tracks from the intro is not good) which is especially useful when listing on all songs shuffle when I'm driving (I have a Parrot car kit which shows the artwork on it's colour screen) so I can quickly skip forward a track.
What I've found is that the first track on any albumn seems to always have the artwork (as described by others) but all others don't, unless I I either switch the phone to landscape and back again or flick back to the albumn details and back again, but this doesn't always work. When viewing my library in iTunes, every track has artwork, so it didn't seem to make sense that artwork was missing on my iPhone.
On reading some of the replies here, I have taken to simply playing each albumn in turn (whilst at home with fast broadband) skipping through each track. You'll notice the wifi wheel kick in when you play an album/song for the first time.
Once I've skipped through each track, I check the album again and all tracks now have the artwork associated and when you play the album later the phone doesn't show any wifi activity. If activity is shown you'll probably come across a track still missing it's artwork!
Whilst time consuming if I was to do my entire library, it seem to provide a solution to the missing artwork on an iOS device.
To me, it seems that the association between artwork and all tracks in the same album hasn't been fully made until you play them on an iOS device (running iOS5).
When adding any new music I will take that extra minute to manually edit the entire album (selecting multiple tracks) an re-associate the artwork as suggested by many others in the hope that this will prevent this occuring, but I'm not entirly convinced.