If Bitcoin is ratified as a currency would its code be BTC?
The normal rule is (two letter country code) + (letter), although that's arguably already been broken by the Euro having EUR instead of EUE.

BT is the country code of Bhutan, whose currency has the code BTN. If anyone will object to BTC for Bitcoin, it'll be them.

According to wikipedia, which is never wrong, multi-national currencies use the X prefix. XBC is taken by "European Unit of Account 9", but XBI is available.

http://en.wikipedia.org/wiki/ISO_4217