Today's the day! Dark Horse is now available on both DVD and Blu-Ray from Virgil Films, and the same from Mongrel Media in Canada. All are quite bare-bones (what, did Criterion spoil you?), the menus only sporting Play and Chapter options... not even a trailer or optional subtitles. But it looks great, as any new release should, and it's a terrific film. I mean, granted, this is the guy who runs toddsolondz.com recommending a Todd Solondz film to you, so you should realize I might be a bit predisposed to appreciate it; but for me, it's one of his best films in a long time.
Dark Horse is also available on DVD (and has been for a few weeks) in the UK from Axiom Films and Australia from Reel DVD (pictured). So that's your Regions 1, 2 and 4 covered. The Australian disc even includes the trailer, edging it just ahead of its domestic counterparts. There are no Blu-Ray versions available from Axiom or Reel however.