I added the memcache module to my Drupal this morning.
Mine is a stand-alone install, so I can test it without affecting all the other ones, that run via a shared codebase.
It requires an updated PECL extension (the default one in Hardy is too old and doesn't support multiple memcache servers) and some code patching as well. The latest version listed in the patches/ subdir is Drupal 5.8, but it runs fine on 5.14. Sofar anyway. If you can't read this, it's possibly broken ;-)
Twitter seem to have modified their RSS feed, which has broken my Aggregation based Twitter sucker. Aggregation itself needed an update, but since the only thing I used it for stopped working I actually just turned it off instead.