Drupal clustering has been on my mind of late. (Apologies for not linking to the few mentions I found but the DirecWay connection is up to its no-good again.) There are a few different things that I ultimately mean by clustering. Perhaps the most extravagant gets pretty far beyond what a community/content management system should do.
The recurring dream is that I can publish anywhere, or at least to any of the Drupal sites I have permissions on, and do it easily from a single back-end. When I write something that is suited for multiple sites I click on the distribution list and the content is published to the various sites. Of course it would be great to have some additional content types like outlines and to get the project module whipped into a great task management system (maybe it should be separate from project... a topic for another day).
Shorter term the idea of having multiple Drupal installations, perhaps one on each machine, all talk to each other is very appealing. Offhand it seems some sort of GUID has to be generated for nodes published in such a system. But if I setup two Drupal sites as peers then I'd like to be able to "publish" locally to the remote database and when a connection exists have it uploaded to the remote master database. I've thought about trying something like this and just making sure that I didn't duplicate node IDs but that is not scaleable.
Some discussion has been online about a separate application install that reads from a read-only database and then publishes to a master. This has a couple of problems. One is dispatching the need for the watchdog table that records errors. So errors and problems would still be written to a remote database which doesn't address the performance issues I'm thinking of. Ultimately I'd like to be able to take notes on a disconnected laptop in a meeting (yes we do still have disconnected meetings) and then upload them (preferably automagically) to the public server. In the other case I'd like to have the home server, on the terrible DirecWay connection, download the database updates (i.e. new nodes, comments etc.) once a day and not be trying to run replication of the database over the pokey-hokey connection.