Solaris is the answer?
Sun have Solaris working on the 32 "core" Niagara (depending on how you count; 8 4-way cores)
See http://www.sun.com/processors/UltraSPARC-T1/ - outstanding.
More within my price range is a dual socket-f athlon board with 2 dual core cpus for now, ready to go to 2 quad core.
From a video presentation by Bill Moore on ZFS (see http://www.sun.com/software/solaris/zfs_learning_center.jsp ) I get the impression Sun have been thinking about concurrency for some time in their kernel.
I'm waiting till www.gnusolaris.com stops being alpha and then I should have most of debians tools and the SunOS kernel.