Previously we were only memcaching clustered stuff (loading people's entries), but Whitaker added support to memcache everything, including all the global (non-clustered) queries, like finding a user's friends and colors, finding when they last updated, etc.
If you know Perl and got a few minutes, please review:
Also, warning: this is the first patch where we've been unable to keep the non-memcached version as fast as before. (Well, we could've, but it would've involved massively parallel codepaths, one of which would've bitrot.) So if you're running a loaded LiveJournal-based site and plan to upgrade to this code, be sure you're using memcached.
Whitaker and I have spent quite some time reviewing this, and Whitaker's done a bunch of testing, but it won't go live for a number of days, probably. When it does, though, we expect some impressive performance improvements.