Yesterday afternoon, the United States filed its motion for summary judgment in Coons v. Geithner, the case pending in the District Court for the District of Arizona. The plaintiffs (which include United States Representatives Jeff Flake and Trent Franks) challenge the individual mandate as exceeding Congress's enumerated powers, as well as the constitutionality of the Independent Payment Advisory Board (IPAB).
In its original incarnation, this lawsuit challenged the IPAB on several different grounds. But as the United States's memorandum filed yesterday makes clear, there is only one IPAB-related claim left: that the ACA's IPAB provisions violate the non-delegation doctrine. The Supreme Court has not invalidated a federal statute on non-delegation grounds since 1936, and it has upheld some extraordinarily broad delegations of authority during that span of years. So this claim's chances of success seem pretty long.
You can find the United States's memorandum in support of its motion for summary judgment here. The plaintiffs' cross-motion for summary judgment is due Monday, August 29.