If I didn't know better, I'd say that they was a blatant money grab. </sarcasm>
I'm pretty sure "Ex Post Facto" is dead.
See the post by David Redden in this thread: http://www.connorboyack.com/blog/ex-post-facto-law.
"Secondly, while there’s some evidence that some of the “framers” thought “ex post facto” included civil laws, it’s well-established that the most influential federalists in the late 18th Century understood an “ex post facto law” to mean a law that was 1) criminal, 2) functioned retroactively, and 3) worked to the detriment of the accused. (See Crosskey, The True Meaning of the Constitutional Prohibition of Ex-Post Facto Laws, 14 U.Chi.L.Rev. 539 (1947))."
http://en.wikipedia.org/wiki/Ex_post_facto_law#United_States
This situation was instigated by a successful lawsuit against the FTB. I wonder if another one is on the way. According to this, however, courts have ruled that retroactive tax laws are not considered ex post facto.