From Barrons:

Richard Epstein of the University of Chicago Law School says that while the tax is more egregious than others, there's no precedent to point to that says the scheme is unconstitutional. But this assumes that what Congress has designed really is a tax, says Erik Jensen of Case Western Reserve University Law School.

"Yes, the taxing clause of the Constitution gives Congress the power 'to lay and collect taxes,' and ordinarily a court won't strike down a charge that Congress says is a tax," Jensen wrote in an e-mail in response to our query. "But that's a matter of deference, not principle. The proposals now before Congress aren't anything like business-as-usual taxation, where deference would be appropriate. A charge imposed at a confiscatory rate of 90% on only a few specified people and on only part of their income isn't what the Constitution means by 'tax.' "

Jensen adds: "It's obvious from the way members of Congress are talking that punishment, not revenue-raising, is involved here. Whatever label Congress uses, confiscating a well-defined category of property from a small group of people sounds a lot more like a taking than it does a tax."

I think that's right (and not just because Erik's a colleague). The key to any constitutional challenge (whether under the Bill of Attainder clause or some other provision) will be convincing a court that the AIG bonus tax is not actually a "tax." The problem is that courts are usually inclined to accept congressional characterizations of legislation — though perhaps this provision will be a stretch too far.

Meanwhile, Professor Larry Tribe is reportedly having second thoughts on the constitutionality of the provision.

UPDATE: Calvin Massey thinks there are real equal protection problems with the tax a la USDA v. Moreno.