To be honest I'm not a database guru, but the following are my personal views
on this topic:

1. I was reading somewhere that PostgreSQL was not multithreaded. This can have
a significant impact if you have a busy database server. Multicore CPUs
designed for executing 10s and 100s of threads simultaneously are just around
the corner (Sun's T1 processors). So fine grained threaded applications are the
future.

The following points are more related to OS characteristics, rather than an SQL
database:

2. In regard to SMP machines, BSDs don't scale as well as Solaris. FreeBSD are
doing a lot of work in making their kernel more SMP friendly, but I don't know
how stable it is, or how much progress they made. I would expect NetBSD and
OpenBSD to lag behind Linux and FreeBSD. I'm running NetBSD-current on dual
Penitum3 and I have experienced frequent system lock-ups, which some people
think are attributed to race conditions in the kernel. I think Solaris is the
best OS for SMP hardware. Linux and FreeBSD scale well to a small number of
processors 2-8, but beyond that Solaris would be a much better choice.

I quite like NetBSD, but I think it needs a lot of improvement in regard to
SMP. Maybe adopt a microkernel design? Redesign major kernel subsystems to
support fine grained threading?