Opposing health reform bill based on the notion that individual mandates are unconstitutional is being viewed as the last line of defense for those opposed to the Democratic reform bills.
I am certainly no constitutional scholar, but when the branch of government the Constitution says can pass laws passes a law, I think there is a fairly strong assumption that it is within Congresses right. This is obviously not always the case, but seems a reasonable place to start.
There are obviously others who spend lots of time thinking about such things, and here are two scholars who say the individual mandate is constitutional.
Here is Erwin Chemerinsky's take that it is clearly constitutional...he is the only constitutional law expert I ever met, so I am partial...he used to be a prof at Duke (he is now Dean of Univ. of California-Irvine Law School). Here is another view on the mandate being constitutionally sound by Mark Hall, who used to be a law prof at Wake Forest but who is now at Seton Hall. He specializes in health law. Here is a longer, more scholarly (fully referenced, etc) version of his argument.