The plaintiff-appellants (Thomas More Law Center et al.) have filed their letter brief at the Sixth Circuit in Thomas More Law Center v. Obama. The letter responds to the questions raised eleven days ago by the panel, which concern standing, ripeness, the IRS's enforcement powers, and the distinction between facial and as-applied challenges in this context. The United States's letter brief is also due today, but it has not yet been docketed at the Sixth Circuit.
You can find the appellants' letter brief here.