We implemented MySQL query caching yesturday and the results are quite impressive.
ab -n 100 -c 40 http://blogs.ubc.ca/blog/category/community/latest-posts/
Requests per second: 325.78 [#/sec] (mean)
Time per request: 122.782 [ms] (mean)
Time per request: 3.070 [ms] (mean, across all concurrent requests)
We now have PHP APC opcode caching, content caching with WP-Super-Cache, content compression with mod_deflate and the MySQL Query caching… still alot more tweaks that can be done but this is a good foundation.