Frankly, even if the plugin is available, IMO the GPU drivers are so unreliable that it would be a hell to support and work around all the different bugs for such a limited audience, and I'm writing that on OpenSUSE 12.1 developing in gVim and experiencing random Gnome Shell crashes every 30-60 minutes (shell just shows background image then comes up in 2-3 secs) on ATI drivers. Opensource drivers are stable but have terrible performance with 3D. So even if it runs I don't expect it to be usable on my PC.

That's been my experience with the ATI drivers myself. Nvidia is far far better, but you can't exactly change a laptop graphics chip. I'm using the open source ones just so that I've got a usable system.

Unity does have an experimental plugin for Linux systems, running on NaCl. It's fairly functional, provided your graphics drivers play nice. I definitely wouldn't recommend it for Facebook games, but for teenaged kids it may well suffice.