Amicus briefs are being filed as we speak in Virginia. Courtesy of reader Mark Regan, I have just received briefs filed by (1) the CATO Institute, Randy Barnett, et al., and (2) a non-profit group based in Washington, D.C., called the Young Invincibles. Those briefs will be posted here shortly. I just checked PACER, and no other briefs have yet to been logged on the electronic docket sheet.
UPDATE: Those amicus briefs, respectively, are available here (CATO) and here (Young Invincibles).