I'm running Glitch on OS X Snow Leopard. I'm not a fan of Flash, and block it in all my browsers, but to its credit, Glitch runs just fine with Flashblock -- two quick clicks to enable it and I'm good to go.
so they've talked about MAYBE doing a non-flash port of the game, and there are certain things about the underlying structure of the game that make such a thing less than insanely impossible but it would still be really really difficult and ..
.. and i would say flash glitch didn't work so well with my older mac laptop but then the devs improved the client performance quite a bit and then getting OS X Lion really put things over the top.
I thought I read from a Glitch interview somewhere that although the game client could be ported to something like HTML5, the Glitch developers (Stoot?) were concerned that it wasn't robust or mature enough, hence the decision to use Flash. I believe they also hinted that Glitch very well could be ported to HTML5 in the future, if and when it matures sufficiently.