Health care mandate - unconstitutional or not?

Is it unconstitutional to require people to purchase health care insurance? Folks at the Volokh Conspiracy have been talking about this a lot lately. There is no precedent for such a mandate. Unlike car insurance, which if people don't have, they can just choose not to drive, you'd be breaking the law if the mandate goes into effect and you don't buy coverage. It'll be interesting to see how the lawyers battle this out. It has been suggested that because health care makes up such a large percentage of the national economy, Congress could argue that mandating coverage comes within its power to regulate commerce. Crazy stuff, people.