We are currently trialling SmartOS with Cassandra and have seen some pretty good results (and the mmap stuff "appears" to work). As Rob said, if this is production cluster, run with linux… there will be far less pain.
If you are super keen on running on something different from linux in production (after all the warnings), run most of your cluster on linux, then run a single node or a separate DC with SmartOS, Solaris, BeOS, OS/2, Minix, Windows 3.1 or whatever it is that you choose and let us know how it all goes!
Its quite possible its well tricked out for Linux.
My major issue with Linux has been that its TCP/IP stack is nowhere near as scalable as Solaris' for massive numbers of simultaneous connections. But thats probably less of an issue with a Cassandra node then it has been with the game servers I've built.