Yesterday afternoon, the United States filed is Brief for Appellees in Liberty University v. Geithner. You can access the brief here.
I have yet to read the brief, but there is one thing I noticed from the cover: the brief is signed by Neal Katyal, the Acting Solicitor General of the United States. This is the first document in the various bits of ACA litigation with his name on it, and it seems to confirm what most have suspected: that the SG will be arguing these cases in the Courts of Appeals.
(I say the SG, rather than General Katyal, because Don Verilli may be confirmed before these cases are argued.)