Currently they are windows only, and the official public kick-off isn't until September 12th. Per their statement:
We'd love to do a Mac port. At the moment our partners at Tessella are working on a linux port, and once we've got that in place we'll get various people together and have a chat about Macs. It's always been something we'd like to do, but we can't say just yet when we'll get around to it.
It sounds like it is something they will look at when they have time. Like many startup projects, you have to conserver your resources. Once they get a successful Windows client up and running publically and then a Linux port that works well, then that would be the time to start pushing for a Mac Client. Of course it doesn't hurt to voice support in advance, so that it is part of their planning.