SUGGESTION

You should reserve a few additional characters for database names and document properties, apart from the current ., ,, $, and _. You will likely need them sometime in the future for something that you don't know of now.

This page http://us2.php.net/manual/en/class.mongodb.php lists some "unusual, but valid, database names". I'm sure there is are a few unusual characters which nobody needs anyway, which you could reserve.