It looks like it's available to flash on some Nexus devices, and their own tablet is shipping within the next month or so: http://www.jide.com/en/remixos/devices
I have no idea how open it is to modification in practice, though. I might consider trying it if I can get a proper shell on it (similar to the Android shell you get in an emulator when using 'adb shell'), even if I cannot fork their DE code itself.
Looks like it's available on their custom hardware, but not as software only.
From the bottom of the page:
Disclaimer: Remix OS is only licensed to authorized business partners and pre-loaded on specific product models of those partners;it's not intended for personal use.
This sort of thing is what makes me hopeful for a Linux desktop one day-- that I actually enjoy using. I'm really excited to give this a try.
Does anyone know where to download or when it'll be available? Doesn't seem to be available yet?