To get the bill passed, congress added a provision that bars the IRS from using any collection mechanisms to collect the tax. They can not garnish your wages, seize your assets, take you to court or put a lien on your property. The only thing they can do is withhold any tax refunds you may be due now or in the future and send you bills and nasty letters. (of course with that being said, the law could change at any time in the future and I wouldn't want to owe 20 years of back health care fines when a future congress decides to let the IRS use all the powers at it's disposal to collect them.)
To make it retroactive like that would ensure a BIG fight in Congress. I think I will risk it (not that I think I will have to, because I think the thing will be repealed). But, if there is absolutely no enforcement mechanism, then millions of people simply won’t comply. What a stupid law!