So, for those, who like me, are ignorant of the subject:
The United States Constitution forbids both the federal and state governments to enact bills of attainder, in Article 1, Sections 9 and 10, respectively. It was considered an excess or abuse of the British monarchy and Parliament. No bills of attainder have been passed since 1798 in the UK. Attainder as such was also a legal consequence of convictions in courts of law, but this ceased to be a part of punishment in 1870.<5>
Bills of attainder were used through the 18th century in England, and were applied to British colonies as well. One of the motivations for the American Revolution was anger at the injustice of attainder—though the Americans themselves used bills of attainder to confiscate the property of British loyalists (called Tories) during the revolution. American dissatisfaction with attainder laws motivated their prohibition in the Constitution (see the case of Parker Wickham). The provision forbidding state law bills of attainder reflects the importance that the framers attached to this issue, since the unamended constitution imposes very few restrictions on state governments' power.
Within the U.S. Constitution, the clauses forbidding attainder laws serve two purposes. First, they reinforced the separation of powers, by forbidding the legislature to perform judicial functions—since the outcome of any such acts of legislature would of necessity take the form of a bill of attainder. Second, they embody the concept of due process, which was later reinforced by the Fifth Amendment to the Constitution. The text of the Constitution, Article I, Section 9; Clause 3 is "No Bill of Attainder or ex post facto Law shall be passed". The constitution of every State also expressly forbids bills of attainder. For example, Wisconsin's constitution Article I, Section 12 reads:
No bill of attainder, ex post facto law, nor any law impairing the obligation of contracts, shall ever be passed, and no conviction shall work corruption of blood or forfeiture of estate.
Contrast this with the subtly more modern variation of the Texas version: Article 1 (Titled Bill of Rights) Section 16, entitled Bills of Attainder; Ex Post Facto or Retroactive Laws: Impairing Obligation of Contracts: "No bill of attainder, ex post facto law, retroactive law, or any law impairing the obligation of contracts, shall be made".
Traditionally, most bills of attainder designated the persons subjected to punishment by name. In some cases, however, legislatures imposed punishment on groups whose individual members could be ascertained without much difficulty. For example, a federal statute made it a crime for members of the Communist party to serve as officers of a labor union. The purpose of the statute was to protect the national economy by minimizing the danger of political strikes. The Court in United States v. Brown (1965) invalidated the statute as a bill of attainder. Since not all members of the Communist party were likely to incite political strikes, and since noncommunist agitators might also engage in such conduct, the decision whether the activities of a particular person presented the danger to be guarded against should have been left to the judicial branch.
Up until 2002, only five acts of Congress had ever been overturned on bill of attainder grounds. The Elizabeth Morgan Act was overturned in 2003 as a bill of attainder. Many suggested that the Palm Sunday Compromise in the case of Terri Schiavo was also a bill of attainder. The cases of U.S. v. Brown,<6> U.S. v. Lovett,<7> and In re Yung Sing Hee<8> establish bills of pains and penalties as punishment without trial, and included within the prohibitions of bills of attainder. The precedent that best reflects most of the original intention of the mandates is from Cummings v. Missouri.<9> It states
A bill of attainder, is a legislative act which inflicts punishment without judicial trial and includes any legislative act which takes away the life, liberty or property of a particular named or easily ascertainable person or group of persons because the legislature thinks them guilty of conduct which deserves punishment.
U.S. v. Lovett was a case historically relevant to taking away pay checks of government workers Congress could accuse of being Communists. This was an asset forfeiture case. It states:
Legislative acts, no matter what their form, that apply either to named individuals or to easily ascertainable members of a group in such a way as to inflict punishment on them without a trial, are 'bills of attainder' prohibited under this clause.
This expansive interpretation, however, makes it difficult for the legislatures to settle any individual cases and to pass any private bills, because taking away private property or rights could be construed as "punishment" even when the goal is to meet justified claims of some other party to the controversy.
http://en.wikipedia.org/wiki/Bill_of_attainder