For a really good description of what’s legally going on here, read this New Republic piece. It’s long, but you’ll at least be educated on the legal implications and historical precendent of the constitutionality of the individual mandate.
The key issue here is the meaning of a single word: “activity.” In this reading, all of the past rulings on the Commerce Clause, even those acknowledging its broad reach, refer to the government’s authority to regulate activity. But neither the Constitution nor the judges who have interpreted it ever suggested the government had the right to regulate non-activity—which is a fair description, according to these lawsuits, of a decision not to obtain health insurance. Like many good constitutional arguments, the argument can be put a lot more simply: If the government can penalize you for not buying insurance, can it also penalize you for not buying a television or a GM car? John Yoo, the conservative Berkeley law professor who served in the administration of George W. Bush, makes the argument this way: “The court has never upheld a federal law that punishes Americans for exercising their God-given right to do absolutely nothing. Even the furthest reaches of the Commerce Clause have extended only to affirmative actions, such as growing wheat or possessing illegal drugs. The only counterexamples that come to mind are the draft and jury duty, and those arise from other constitutional duties than Congress’ power over interstate commerce.”