POSIX calls, which have always been there but require a recompiled binary, are different than the binary compatibility announced today. This additional step is actually quite an impressive feat of software engineering. To think that Microsoft would care so much about maintaining compatibility would have seemed ridiculous just a few years ago.
It should be noted that FreeBSD has had this for a decade AFAIK, and LX-branded Zones have been massively improved in SmartOS.