When should an individual or business file a SAR with FinCen?
While bitcoin may seem to be useful as an offshore bank account, it likely won't be subject to the FBAR reporting requirements.

Now if you have fiat (e.g., USD) funds at an exchange that is not in your own country (e.g., Mt. Gox) and those funds exceed $10,000 worth then possibly that could be something where an FBAR filing might be required.

So having $10K or more at an exchange might be reportable but having $10K worth of bitcoins may or may not.  

An excerpt from someone who studies this topic:


  The critical issue is whether a wallet constitutes a financial account
  and if a wallet constitutes a financial account then what constitutes
  signature authority?