The reason behind this post, is that we believe some guys/girls are thinking right now about complex integration - and, if only that, could be refreshing to know that it could be made - almost painless.
After 2 years or so playing with Drupal - started in the 4.x era - we are re-launching our heavily Drupal/GNU-based community. Once a single-site deployment - and meanwhile waiting other software than mere HTTPd's upgrade to database integration - we are now running a multi-site, multi-protocol endeavor.
Our technical goal is to achieve the right balance between isolated areas (i.e. IRC/chat, forum, campus) while at the same time, taking advantage of the terrific integration capabilities of Drupal (i.e. taxonomies); not an easy task, though - but we are having very promising results as far.
Up to date, we're using D5 and D6 (main site will be frozen in D5 until every critical module used has a D6 upgrade suitable to migrate it), in Slackware and Debian-based Linux distros. A Moodle setup coexists (peacefully?) with the rest of the puzzle, as well as IRC (Unreal/Anope), vhosted-HTTP (Apache/LigHTTPd) and streaming (Icecast) server daemons, aiming to get closer to that "totally integrated thing" - granted, we're still trying to figure out how to do certain things, **WITHOUT** breaking core-compatibility and the like, even with custom modules and/or SQL perversions; things may vary in the near future... multiple-site user login will play a big role in that, as well as better 3rd-party integration Drupal projects gains more relevance.
If feel like taking a tour (no demo, the production beast), visit us at: www.aldeacity.com.ar
The very best of success.