The case is United States v. White; here’s the relevant analysis:
Seems pretty sketchy to me. Section 922(g)(9) is literally not a “longstanding prohibition,” since it’s quite recent, so whatever force longstanding tradition might have had in Heller, it doesn’t apply here. And while the possession of guns by violent misdemeanants may indeed be “dangerous,” I don’t think that by itself is enough — even given the analogy to felons — to conclude that the ban is constitutional; all gun possession is dangerous, and all gun bans aim to “address[] the thorny problem of … violence,” including “domestic violence.”
I’m also surprised that the panel opinion doesn’t discuss the Seventh Circuit’s United States v. Skoien decision, which is considerably more thorough in its reasoning, and which remanded the case to the district court for an evaluation of the ban under the intermediate scrutiny standard of review.
For my inconclusive views on the analysis, see this law review article, and especially PDF page 58. Many thanks to Prof. Douglas Berman (Sentencing Law and Policy) for the pointer.