Songs purchased outside of the United States iTunes Store containing DRM (Digital Rights Management) will not be matched or uploaded to iCloud.
Argh, looks like songs that you bought in a non US iTMS before iTunes went no-DRM will not be matched to the iTunes iCloud. I have quite a few of those (living in NL I use the Dutch iTMS mostly), I never bothered to convert them to iTunes Plus. To have them matched, I probably have to convert them to iTunes Plus first, which adds quite a few euros to the bill.
So, I guess I will just download those illegally and have them matched.. WTF are you doing Apple?
I guess I was mistaken. I have 11752 items in my Music library. iTunes Match matched 11388 songs. Difference of 364 songs. I have about 2609 songs still marked as "Protected AAC" - the old DRM-ed files. Looks like those are matched anyway, and the 364 remaining are some obscure items that can't be matched (I don't have any illegal stuff in my iTunes library)