Much ado was made of the Great West v. Knudson case, decided by the United States Supreme Court on January 8, 2002. Not since 1990 had a case on subrogation come from the U.S. Supreme Court affecting a self funded ERISA plan's ability to enforce a subrogation or reimbursement provision.
Until Knudson, self-funded plans were able to pursue subrogation and/or reimbursement under several theories, or “causes of action.” The Knudson decision, however, distinguished those theories and discussed the nuances of "legal" (i.e., contractual) claims versus "equitable" (i.e., wording from the ERISA statute) claims. The Knudson Court concluded that self funded ERISA plans do not have legal claim for subrogation in federal court, only equitable ones. But even the Knudson Court said "no" to Great West's equitable claim because of how that case was plead, unusual facts, and a complicated procedural history. The fact that settlement funds in the case were previously distributed to a Special Needs Trust also made a difference in the outcome. Confusion set in.
Then came Sereboff v. Mid Atlantic Medical Services, decided in early 2006, after the Sereboff's previously refused to reimburse their health plan's $75,000 lien from a $750,000 settlement. The decision cleared up much of the gray area created in Knudson. The Court affirmed a plan's equitable right to subrogation pursuant to plan terms. It also went as far as saying that plans have an equitable lien by existence of their subrogation provision even before recovery funds have been received, although it is still important that those funds are identifiable and not dissipated (like in Knudson).
For more insight, and some interesting tangents, see Wikipedia's entry.
Remember that Sereboff supports a plan's right, but it doesn't guarantee it. Actual recovery depends more on early case identification, the right plan language, and systematic execution.