Equally true, no case has ever found such a tax (likely due to lack of precedential examples) unconstitutional,  and several other tax schemes have withstood BoA challenges and dicta in some of  those case makes it seem  a tax would need to be extraordinarily targeted and punitive for BoA analysis to apply.

"Courts sometimes look to the motivation of the legislature, and this looks like an intention to punish," said Steve Johnson, a former IRS lawyer who teaches tax law at the University of Nevada, Las Vegas. "It could also be viewed as government confiscation."

Lawrence Tribe seems to think the obviously punitive intent of the bonus tax pushes it over the line into being a bill of attainder. Here, for example...

I just got off the phone with Harvard professor (of Constitutional law) Laurence Tribe, who advised Obama during the campaign, and he says he's leaning towards seeing the new House bill to tax back all the AIG bonuses as unconstitutional.
Tribe says the problem with the bill is that the Constitution forbids Congress from enacting a "bill of attainder," which would essentially "legislate punishment of an identifiable class," as he put it. Tribe noted that the Supreme Court had used that clause to slap down other laws.

Tribe says the main problem is that it's hard to make the case that the law isn't "punitive."

"Its punitive intent is increasingly transparent," Tribe says. "when you have Chuck Grassley calling on [executives] to commit suicide, and people responding to pitch fork sentiment, it's hard to argue that this isn't an attempt to punish an identifiable set of individuals who are the subject of understandable outrage."

The whole point of opposing bills of attainder, Tribe says, is to prevent what some have called "trial by legislature."