The development version of FreeBSD ( 7.0 ) seems to scale a lot better then linux on SMP systems.
A combination of latest freebsd scheduler ULE 2.0 that is built into FreeBSD 7.0, the libthr threading library and a patch (not commited, yet ? ) that addresses poor scalability of file descriptor locking and some other patches is what made a system with 8 core amd64 cpu and 16 GB of ram outperform linux by a factor of 4 in MySQL tests.
The tests were performed using sysbench OLTP benchmark, a tool designed for testing mysql performance.
Brief test results: Linux is actually a little ( ~ 2% ) faster then FreeBSD for less then 9 clients, but when the number of clients grows to 20 linux performance drops a lot and FreeBSD's performance stays the same.
More details about the test here: