I was surprised (besides being delighted that all the extras were subtitled and/or captioned) when watching WALK HARD the other night to find that the movie's audio-commentary has been Subtitled in English.
I know of a handful of Asian movies that had their commentaries subtitled in English for US release and I recall that the LORD OF THE RING movies would flash up the name of the person talking while the commentaries played, but I can't think of another movie where the commentary was subtitled like this. I wish more DVDs were like this, however - it feels like the producers/studios/whatevers feel deaf people have no interest in the making of movies when they don't bother with captioning/subtitled extras, and that includes the commentaries.
Oh, wait! Doesn't one of the DVD releases of MONTY PYTHON AND THE HOLY GRAIL also feature a subtitled commentary? I recall in that case, it's more of a joke commentary for people who "don't like the movie" however.
Still, any others? I'd be curious to check some out if there are any.
In the UK, commentaries are frequently subtitled. Most Sony and Universal releases include subtitled commentaries. All the commentaries on the recent BLADE RUNNER box set are subtitled.
BFI's DVD of THE INNOCENTS has an English language commentary with English subtitles (as well as subs for the feature itself).