Songs purchased outside of the United States iTunes Store containing DRM (Digital Rights Management) will not be matched or uploaded to iCloud.
(emphasis mine)
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?
[edit]
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)