As promised for today a new version of the multi-db package is available (in beta form).
This new version is much different from the previous version. It's basically a hybrid of the old version and HyperDB.
This new version contains:
1) Support for replication
2) Support for multiple DCs
3) Support for multiple VIP databases
4) Less complicated configuration (hopefully)
5) Better performance (at least from my testing)
Obviously a lot has been changed so if you have a free minute and the resources please give it a whirl and let me know about any problems.
I have intentionally left out the instructions to hopefully limit this beta only to the experienced users. For this release in place of instructions there is a sample config file. Simply modify it to your needs, rename it to db-config.php and drop it in wp-content.