Re: WordPressMU on multi-DB do people use?
You need to ask that question on the Premium WPMU site. It is a 3rd-party project not supported by Automattic.
If you want a free multiple database engine, you can use HyperDB. This site runs on HyperDB. WordPress.com runs a highly-modified version of HyperDB.
Here are a series of articles written by Trent Adams, one of the BuddyPress moderators, that detail how to configure HyperDB: