According to a poster with insider information, on the Pro Tools Forums:
It's my understanding that the most recent versions of OSX have departed from the Unix kernel heritage.
I have heard from a reliable source, inside Digidesign, that they actually have Protools running on Linux, and that the port from OSX isn't that hard, but are under contract obligation to Microsoft to not release a Linux port. Otherwise Microsoft can revoke their access to the Windows SDK.
Justice C. Bigler
House Sound Technician
Tulsa Performing Arts Center