Recent publications by the US federal government indicate that Bitcoin will likely be used in illicit cross-border transactions, and that the Office of National Drug Control Policy (ONDCP) is ramping up efforts to detect and interdict bitcoin transactions. There is a special focus on Canada and the United States.
http://letstalkbitcoin.com/blog/post/united-states-to-monitor-canadian-border-for-illicit-drug-related-bitcoin-transactions
I am not sure exactly how they plan on monitoring bitcoin transactions like this. Although it is, in theory possible to determine the country of the person sending a TX, it is impossible to determine the country of the person receiving, especially if the address being sent to has never been used http://letstalkbitcoin.com/blog/post/united-states-to-monitor-canadian-border-for-illicit-drug-related-bitcoin-transactions