well, the next obvious question is --- the youtube ID still unique?
if so, the domain/sub-domain wouldn't matter anyway..
no ?
example:
http://ca.youtube.com/watch?v=6hoiRLzqhughttp://www.youtube.com/watch?v=6hoiRLzqhugsame vid -- so i see no harm in just eating the URL up and using markup to the most logical sub-domain for the board..
only real issue might come in where the board is all over the map and diff sub-domains might be diff for diff users
if you get my drift..