United States persons are required to file an FBAR if .. . The aggregate value of all foreign financial accounts exceeded $10,000 at any time during the calendar year to be reported.
http://www.irs.gov/Businesses/Small-Businesses-&-Self-Em...
However, if I have my bitcoins here in the US, does that count as a foreign financial account? Or is it more akin to having a local store of gold. I doubt the IRS has any real guidance for these questions yet...
Once the IRS actually gets to work and addresses this they'll probably end up altering terminology so that a bit coin "account" is comparable to all bit coins within an individuals direct ownership and control.
In the end investment proceeds from bitcoin will be taxed and traced. I wouldn't try to outsmart or game the US government in terms of account balances, I could see a "smart" early adopter going to jail because of their efforts to avoid taxes.
You would certainly get a significant rise in tax evasion. I'm not actually convinced that's a bad thing overall (though I'm not inherently anti tax). If it brings tax schemes previously only available to the wealthy into the hands of commoners.
Ask any Argentinian if they'd like to have kept a stash before the Corralito.