This one will be nice and dry.
The House passed, and it looks like the Senate will or has (too lazy to look it up right now) a bill that will tax bonuses to employees of companies that took bailout money at 90%. Basically, between that and the tax bracket the bonus will be voided. Obviously the motivator for this is the AIG bonuses, but it applies to any company that takes bailout money.
There are various arguments about the unconstitutionality of such an act by Congress. Not having any sort of legal background, I have no insight into how valid these arguments are; however, I did come across this today from someone that does have a law background. The comments, particularly about the 5th one in or so (the start of a thread of comments) are pretty helpful.
It seems that the constitutional question comes down to whether or not this is a tax or a punishment. If it is a tax, it seems that the various arguments wouldn't apply ("Bills of attainder" and "ex post facto"). However, if it is a punishment it would be a problem.
That said - assuming I understand the arguments - since this is aimed at income and not property, and because it is being attached as a condition to government aide (in the same way non-profits have to deal with retroactive rules), this doesn't fall outside of the constitution.
Either way, it is probably heading to the courts. Whether or not it is helpful financially to do this, I still maintain that Congress has to do something to satisfy the people and this is along the right lines. It is all about perception and right now the perception is these guys are on the wrong side of it.