It's strange, because I use a lot of non US-ASCII characters (even my commenters do), and in my testing period I've tried even other alphabets than the Latin and it worked perfectly!
Are you by any chance copying your posts from a word processor that encodes the text differently than utf?
Or are you using any of the textile/markdown plugins? I remember some posts about those screwing up the characters...
If I find them I'll post back.