According to developer Marty Stratton the Mac and Linux versions of Quake Live have "proved more difficult than expected, but we're getting close. We expect to also be testing Mac and Linux versions of QUAKE LIVE internally this month and then making those publicly available just as soon as we feel they are ready. This work is being done by a separate programmer in parallel with the other work that we're doing, and is his only priority - point being, that this is a top priority for us and not being delayed because of other work."
Quake Live (formerly known as Quake Zero) is a first-person shooter browser plugin-based video game developed by id Software, which was first announced in 2007. It is a browser-based variant of its predecessor, Quake III Arena (Q3A). The game entered an invitation-based closed beta in 2008, and an open beta began on February 24, 2009. [Wikipedia]