According to the IRS commisioner, other than Tax refunds they have no way of enforcing this.
http://dailycaller.com/2010/04/05/irs-c ... ax-refund/
Speaking at the National Press Club on Monday, Shulman downplayed the IRS’s role in enforcing the recent overhaul of the health insurance industry by claiming the agency would not aggressively target individuals who don’t purchase coverage. He noted that the health-care bill expressly forbids the agency from freezing bank accounts, seizing assets or pursuing criminal charges, but when pressed said the IRS would most likely use tax refund offsets to penalize those that don’t comply with the mandate. The IRS uses refund offsets to collect from individuals that owe the federal government a delinquent debt.
How long before people realize they can just wait until they need insurance to go buy it? You know damn well if the system can be gamed people are going to take advantage of it and this loophole will allow people to not get coverage or even drop their existing coverage if they please.