[Erwin]

Some simple ones off the top of my head:

1. Multiple webservers not able to make use of file-based datastore (the admincp server will create the correct one but other servers will have an outdated one).
Solution: Use memcache and have a central memcache server (now supported by vB3.5.x). My solution is to use eAccelerator to cache datastore by enabling it in config.sys, and then each time datastore is updated (bitfield rebuilt) I restart eAccelerator using its web interface - this renews the datastore in cache. Works like a charm and reduces internetwork traffic and db query at the same time.

2. Multiple webservers mean that for attachments and uploads they need to go to the one webserver.
Solution: Hack attachment.php so it is always read from the one subdomain from the one server and uploads go to that one server, then change templates and style info to specifically read all images from that one server.

3. Search kills database server.
Solution: Set up a slave search server by replicating just thread, post, forum and user tables using mysql replicatiion, and hacking search.php to read from this slave server instead of the main server. I have also hacked the mod usertools.php for searching ip to read that slave server for ip address searching as well.