"an unprecedented encroachment on the sovereignty of the states" (quoted verbatim from the legal complaint)
It's gone from speculation to fact as the attorneys general of Florida, South Carolina, Nebraska, Texas, Michigan, Utah, Pennsylvania, Alabama, South Dakota, Louisiana, Idaho, Washington, Colorado, and Virginia sue. (To be nitpicky, though, I could point out that technically Pennsylvania and Virginia are "commonwealths.")
Those of you gentle readers who are legal eagles might like a look at the complaint (in PDF).
Here are a few more thoughts about the individual mandate and judicial review at Volokh, a law professor blog.
A thought: the fact that more than 30 states are asserting their opposition to ObamaCare is something worth remembering.
(I feel that I should create a tag named "states' rights" or something like that, but that term kind of sounds too Civil War-ish to a history nerd like me. Can you think of a better tag?)