The Supreme Court ruled that this law was constitutional because this penalty is a tax. They ruled that Congress cannot mandate that people buy health insurance, but that they can tax you if you do not. They ruled that this is a tax and not a penalty because the tax is so small. Vertigo has mentioned a figure around $95 several times in the forums. Now obviously $95 is a lot less per year than it would cost to buy health insurance. The Court did say, however, that if the tax was so high and burdensome that it would basically ruin you if you did not buy health insurance, that would be unconstitutional since that would be forcing people to buy health insurance via the tax code. Here are some more key facts: the bill has several features which are meant to protect people from not having insurance such as no longer allowing insurance companies to deny coverage based on pre-existing conditions. One more thing, the IRS is not allowed to use aggressive methods in collecting the penalty/tax for not buying health insurance.
So, what's the loophole? Don't buy insurance, don't pay the penalty, and then only buy insurance when you get sick since they can't deny you coverage. The penalty/tax is so small that it's a lot cheaper than buying insurance, and the IRS is likely not going to be able to collect it from you anyway. Not only that, you can wait until you are sick and then buy insurance and be fully covered. And here's the kicker: since the Court ruled that the law is constitutional only so long as the penalty/tax remains small enough to be considered to not force people to choose to buy health insurance, they can't raise it without risking the bill being overturned!