I won't say too much on the subject—because I just took the relevant constitutional exam last week and I am not confident I did well—but I can say that Democrats were not stupid not to include a severability clause in the health care bill. That's the silliest thing I've heard yet in the health care debate (and people are arguing that the decision to rely solely on emergency room visits for your health care needs is not an economic activity!). First, the health care bill would not make sense without the individual mandate—it relies on a bigger insurance pool to reduce costs for insurance buyers. There was no reason for Dems to include a severability clause because they didn't and don't want the bill to exist without the individual mandate. Second, courts avoid dismantling central components of laws whether or not there's a severability clause spelled out in the legislation.
Both pro- and anti-health care decisions will be appealed all the way to the Supreme Court, so this lone federal judge doesn't really matter. (But Justice Kennedy does! And maybe even Justice Scalia.)
UPDATE: Actually, it turns out that the judge thought the provision WAS severable, which means the doofuses at TPM that Dan linked to were scared of nothing, but it also means I overestimated the judge. The judge struck down only the individual mandate provisions; he left the rest of the law standing. (pp. 38-40)