Physician Hospitals of America v. Sebelius is the case currently pending before the Fifth Circuit. At issue is ACA section 6001, which limits the plaintiffs' ability to bill for services to Medicare patients who were referred by a physician-owner of a hospital. The United States prevailed below in the Eastern District of Texas (you can find the district court's opinion here). The physicians' constitutional claims are that section 6001 lacks a rational basis (and thus violates the principles of due process and equal protection), amounts to an impermissible taking of property without just compensation, and is unconstitutionally vague.
The challengers have filed their brief on the merits. The United States' brief as appellee was originally due October 5, but the Fifth Circuit has granted the government an extension until October 19.