I don't know enough Constitutional law to judge the merits of this, but some people who do clearly think that the insurance mandate in the health care bill may be unconstitutional. Let's just imagine for a moment that it fails a constitutional challenge. What happens next?
Well, what happens next is a disaster of epic proportions, because the rest of the bill would still stand. So with the provisions preventing insurance companies from denying coverage of pre-existing conditions, there would be no reason to have coverage until you needed it. So healthy people would stop buying coverage, and the pool would be only sick people. Premiums would skyrocket; sick people would go bankrupt; insurance companies would go out of business. It would basically mean the regulatory destruction of an industry.
Now, most likely this sequence of events wouldn't actually occur because, if the mandate is overturned, there would be some time to legislatively repair the damage before the pre-existing condition provision comes into effect. But the mandate is the linchpin of the whole bill. I'm really not sure how any of the rest of it stands without it. It would be extremely interesting to see how the debate on how to fix things afterwards would play out.