After a few days of experimenting with configuration, and about a day running one server as a test, we've now got 13 web servers online and serving pages out of the batch delivered last week.
These are spiffy Opteron boxes, nice and fast, and should help take the load off the other apache boxen quite nicely.
For now the Opteron apaches are running with APC as the opcode cache rather than the Turck MMCache we're using on the Pentiums. We used to use APC a couple years ago and it was a bit flakier then, but the current version seems to be much more responsive with performance in line with the competition* and is being actively maintained to work with current versions of PHP.
* http://meta.wikimedia.org/wiki/Turck_vs_APC_benchmark
-- brion vibber (brion @ pobox.com)