Voiced and Unvoiced /th/: Is there a rule?

(Grammar Questions I Couldn't Answer)

This isn't actually a grammar question--it's more of a pronunciation question.  But since this question has to do with the grammar of pronunciation, I'm counting it here.

When teaching the voiced and unvoiced /th/ sounds to my students (/θ/ and /ð/), one of the students asked if there was any sort of rule as to when /th/ was voiced and when it was unvoiced.  I said I didn't know.

I later asked my manager, and  he pointed me to this Internet thread, which shows some discussion about the rules for voicing /th/ sounds.