Congressional Subpoenas And State Court Orders:
In a last-ditch effort to block a state court order mandating the withdrawal of a feeding tube from Terri Schiavo at 1 p.m. today, the United States House of Representatives is issuing a Congressional subpoena "requir[ing] hospice administrators and attending physicians to preserve nutrition and hydration for Terri Schiavo to allow Congress to fully understand the procedures and practices that are currently keeping her alive."

  I am no expert in the law of Congressional subpoenas, but I can't see how Congress can justify this order as being within the scope of the Congressional subpoena power. The subpoena power is an investigative tool: it permits the issuing authority to compel individuals either to come testify before the issuing authority or else to bring physical items such as documents and records to the issuing authority for inspection. The key mechanism is the power to compel the disclosure of evidence to Congress so that Congress can understand the facts and craft legal solutions accordingly. The power to compel disclosure of evidence doesn't seem to have much to do with the power to block a state court order requiring the removal of a feeding tube.

  UPDATE: A story in the New York Times suggests that Congress is attempting to justify the subpoena under the witness-tampering statutes:
Bill Frist, Republican of Tennessee and the Senate majority leader, issued a statement saying that the woman, Terri Schiavo, and her husband, Michael, were being invited to testify in a Congressional inquiry into the matter later this month.

The statement pointed out that Federal law protects witnesses called before Congress "from anyone who may obstruct or impede a witness's attendance or testimony."
I don't think that works, though. Even allowing the legal fiction that Terri Schiavo is expected to testify next month, the fact that witness tampering is a crime does not give Congress the power to identify something that it sees as potential witness tampering and then issue an order enjoining that from happening. Perhaps I'm mistaken, but my tentative sense is that this is far beyond the scope of Congressional power.