I did a quick google and found a discussion of retroactive tax laws (applying to gift taxes in 2010): http://www.assetprotectionsociety.org/the-likelihood-and-enf... Potential arguments include due process, "wholly new tax", lack of notice, ex post facto (with the same caveat as mentioned here about applicability only to criminal cases), and the fifth amendment taking clause. Conclusion: it's hard to fight.
> applicability only to criminal cases Where do you get that? The Constitution says simply: "No Bill of Attainder or ex post facto Law shall be passed." Period, end of story. Just because someone on the Internet says this applies only to criminal law doesn't make it so.
See Carpenter v. Pennsylvania, 58 U.S. 456 (1854):
"The debates in the federal convention upon the Constitution show that the terms "ex post facto laws" were understood in a restricted sense, relating to criminal cases only, and that the description of Blackstone of such laws was referred to for their meaning. 3 Mad.Pap. 1399, 1450, 1579.
This signification was adopted in this Court shortly after its organization in opinions carefully prepared, and has been repeatedly announced since that time. Calder v. Bull, 3 Dall. 386; Fletcher v. Peck, 6 Cranch 87; 33 U. S. 8 Pet. 88; 36 U. S. 11 Pet. 421."