So we got some exciting new data. I realized that we've been running with mysql 5.0.27 on my machine. I updated and re-ran the tests. I also ran with a 22.214.171.124 kernel on linux. The 'contention' refers to a bundle of contention reducing patches that are being tested for stability and inclusion in -CURRENT soon.
I would like to caution everyone against getting too excited before we finish verifying all of the results. I'm talking to some linux kernel guys now who are trying to reproduce this effect. Until then, I'm glad to see that with the same version of mysql we get the same throughput at peak. I have some scheduler changes in the queue that should address the slower ramp-up on freebsd as well.