I finally got around to taking a quick look at memcached. It strikes me as quite elegant, and ought to be fairly easy to drop in in a number of places. The PHP client library is GPL, and the server is BSD:
http://www.danga.com/memcached/
At the moment we don't exactly have a lot of free memory floating around to work with, but hardware upgrades are on the horizon.
Memcached is essentially a really big associative array. :) Rather than shared memory, clients talk to the server(s) by TCP/IP... but this means you can tie together multiple servers (for *lots* of RAM-based storage) and multiple clients, should we in the future end up using multiple front-end web servers.
-- brion vibber (brion @ pobox.com)