[T]he law counts on most of the scofflaws turning themselves in. If you do not have insurance and think you owe the tax, then you will be asked to check a box to that effect on your tax return. If you choose to ignore the mandate, you might also choose not to check the box. But even those who do confess that they do not have insurance may not be liable for the new tax. Illegal aliens, Native Americans, prisoners, those who are without insurance for less than 3 months, those who do not have to file an income tax return, anyone who faces a hardship or cannot find affordable coverage, and others are all exempt.
Joseph Antos and Michael R. Strain at The Health Care Blog. Full post worth reading.