After some investigation, this does not seem to be an issue when the subject/body begins with a latin character. When beginning with a “special” (Russian, Japanese, Greek, etc) character, it seems to fail.
I’m having the same trouble with it. The body of mine is also base-64 encoded UTF-8. I’m going to poke around wp-mail.php and see if I can find a header telling me whether or not I should run base64_decode() on the body.
