Hi everyone, if you have troubles getting back to your music after the update this has helped me out. I didn't want to restore and this is a how to without a restore. (no backup or large itunes match library with slow connection)
The issue on my iPhone 4 with iTunes Match after an OTA update was that there were no actual songs for albums/artists in the library and iTunes Match was stuck on the cloud icon when trying to activate.
1) turn off iTunes Match in both music and iTunes store settings (can be sometimes tricky as turning one off toggles the other)
2) install the iExplorer software (demo is sufficient)
3)connect your iPhone and delete the folder /media/itunes_control/itunes
4) now you will need to reboot (holding home and sleep until apple pops up)
5) You will now have an empty library
6) Try to re-download some of your old purchases or free songs. They will appear in library
7) reboot again (just in case)
8) Turn the Itunes Match on - it worked for me on a second try...
9)You will have all your music BUT old music will eat up your space
10) turn Match off again and shut down the music app
11) Connect phone to iTunes and navigate to music tab. Your old music will be there, play some songs to ensure they really exist on the phone
12) Run a sync and hold on, iTunes might be sluggish for a while, let it finish.
13) Your old music should be back on in the music app.
14) you can turn on the iTunes Match again on, it should load in a minute
15) check the free space on your iPhone - it might be possible that you have more free space that before updating to iOS6
the last point - I went from around 3 GB free to about 10 GB Free and I have all of the old music still on my phone - I didnt delete or redownload anything. This makes me believe that the music app - and more probably the iTunes Match - corrupt some files (remember unfinished downloads, ghost files etc..), making your memory stuck somewhere and thus breaking your Music library during the update....
Please let me know if this had been the case for you! I am very happy now
Just found some 7 GB laying around 

The issue on my iPhone 4 with iTunes Match after an OTA update was that there were no actual songs for albums/artists in the library and iTunes Match was stuck on the cloud icon when trying to activate.
1) turn off iTunes Match in both music and iTunes store settings (can be sometimes tricky as turning one off toggles the other)
2) install the iExplorer software (demo is sufficient)
3)connect your iPhone and delete the folder /media/itunes_control/itunes
4) now you will need to reboot (holding home and sleep until apple pops up)
5) You will now have an empty library
6) Try to re-download some of your old purchases or free songs. They will appear in library
7) reboot again (just in case)
8) Turn the Itunes Match on - it worked for me on a second try...
9)You will have all your music BUT old music will eat up your space
10) turn Match off again and shut down the music app
11) Connect phone to iTunes and navigate to music tab. Your old music will be there, play some songs to ensure they really exist on the phone
12) Run a sync and hold on, iTunes might be sluggish for a while, let it finish.
13) Your old music should be back on in the music app.
14) you can turn on the iTunes Match again on, it should load in a minute
15) check the free space on your iPhone - it might be possible that you have more free space that before updating to iOS6
the last point - I went from around 3 GB free to about 10 GB Free and I have all of the old music still on my phone - I didnt delete or redownload anything. This makes me believe that the music app - and more probably the iTunes Match - corrupt some files (remember unfinished downloads, ghost files etc..), making your memory stuck somewhere and thus breaking your Music library during the update....
Please let me know if this had been the case for you! I am very happy now