Over the weekend I updated SharDB to allow installs to upgrade from WordPress MU 2.9.2 to WordPress 3.0.1.
I also did an initial pass at support for new WordPress 3.0 -> WP network in SharDB. In this implementation the main site’s tables will be in the global database. You can easily set up a test install (to test your plugins, etc. with SharDB). Set up a fresh install that has the network enabled, create your database shards, then configure & install SharDB.