Kyle,
Playing devils advocate, a text file with no byte order market (BOM) is assumed to be UTF-8. In theory if string encoding were determined by BOM in BSON, like it would be for a text file, nothing would break.
Explicitly I'm suggesting that for UTF-16 and UTF-32 a BOM be placed in front of every string value in a JSON document.
Now, I'd never make use of such a feature, and I agree UTF-8 is the way to go. I also realize this might lead to the evil UTF-8 BOMs getting added into "normal" bson documents. However, if some people want to do it, I say let them.
Justin