They already have - see opensolaris.org .
There are some iffy parts about the license they are using that have many in the OSS community skeptical:
“Because the terms of the CDDL license do not permit CDDL code to be used with GPL code,” she says, “nothing in these patents can be used with Linux. Sun thinks that if it just releases its OS, suddenly the open source community will decide to write open source code that won’t work with GPL. I don’t understand what these guys are thinking.”
This is pretty silly as you an use GPL with BSD or any number of licenses but not the CDDL so much of the opensource software out there is already on shaky ground when it comes to working with SUN.