There is a new law, the Foreign Account Tax Compliance Act (or FATCA), that was passed in 2010, and will go into effect on 7/1/2014 and be required to be adhered to for the 2014 tax year. (see
http://en.wikipedia.org/wiki/Foreign_Account_Tax_Compliance_Act). It was added as an amendment to the Hiring Incentives to Restore Employment (HIRE) bill that added tax incentives to companies that hired employees that were unemployed previously.
FATCA was created as an effort to close some of the tax "loopholes" of investors putting their money in foreign financial institutions (FFI). FATCA would require FFI to disclose to the IRS information pertaining to any US citizens (including foreign residents) investing in these FFI, including the person's tax ID number (TIN; for individuals, this is usually just their Social Security Number), the person's address, financial accounts with the FFI, etc.). The FFI will have to deal with all of the expense that this entails, or if they decide not to comply, then a hefty 30% punitive tax will be imposed on the FFI for all US asset withdrawals. It also requires US citizens to file a new tax form along with a 1040 to the IRS or face possible 40% understatement penalties.
Although no one can really say what effects this law will have on the foreign financial markets, some of the criticisms that this has received appear to already be making itself felt. Because of the 30% tax on non-compliant FFI, some are refusing to accept new accounts from US citizens, and may even lead to further divestiture of the US market (meaning these FFI may sell of US investments and assets before FATCA goes into effect, which ends up hurting US businesses that had originally sold these investments to foreign markets). Some websites are using this possible effect to portray a complete collapse of the US economy, but while divestiture of US investments in foreign markets may hurt the economy, it probably won't bring about a complete collapse of the economy.
Another issue is that US citizens are revoking their citizenship, esp. foreign residents that don't plan to live in the US (eg people working outside the country, those with dual citizenship, etc.). There has already been a surge in people revoking their citizenship following 2010, and some of the cited reasons are due to FATCA regulations (see
http://www.forbes.com/sites/robertwood/2014/02/06/americans-renouncing-citizenship-up-221-all-aboard-the-fatca-express/). This is very reminiscent of what happened in Ayn Rand's novel
Atlas Shrugged in which the country's most successful industrialists, under increasingly aggressive regulations from a heavily socialized US government, decide to pack up business and move to countries with more favorable business laws. Because of these similarities, some right-wing blogs have too used FATCA as an attempt to scare people into thinking that the US will devolve into the dystopian society posed in Rand's book.
Finally since this law is targeted to any US citizen regardless of residency, there might actually be US citizens that don't realize that they are in fact citizens of the US, which could cause havoc for both the FFI that they have accounts with as well as with the IRS. Because a person is considered a US citizen if they either were born in the US or if they have US parents, some people may not realize that they are US citizens (eg their parents took a trip to the US and birthed their child there instead of in their home country (like as happened with President Obama), or were birthed by US parents but adopted out to a family in a foreign country), which might lead to FFI to dig deep into the past of all of their account holders in order to prevent any accidental non-compliance. Because these FFI may do intensive background checks, it begs the question about a person's privacy, esp. for those citizens in foreign countries.
I'm not sure if this is what you were hearing about, but I do think this is another example of the US being penny-wise / dollar-foolish... In an attempt to try collecting taxes that some shelter in off-shore accounts, the impact of these new regulations may havee serious, unforeseen consequences that will do more harm to the economy then not collecting those additional revenues would produce.
That being said, the new FATCA regulations aren't really about the US currency / the dollar bill. There was indeed a prank sent out for April Fools Day in a financial blog that stated that several of the US border states were going to move to Canadian currency in order to boost tourism. It was a very well-written blog that cited several reasons for this move, and it was only at the end of the article did it state that the entire article was just an April Fools prank. However other blogs were quick to link to this article, either intentionally by thinking it a great prank or unintentionally by not reading the entire article, and so the news quickly spread around the internet. If this is what you were wondering about, then bhiett was indeed correct that it is only a prank, nothing to worry about.