[HyperDB] advice on HyperDB partitioning?

[Jim McQuillan]

We are looking to migrate an existing proprietary system of about 25,000
blogs to Wordpress... either Wordpress MU or more likely Wordpress 3.0
once it is released.

My research into performance and scalability has led me to HyperDB.  I
wanted to see if I could get some advice on the database partitioning. 
To start with we were thinking of splitting the blogs into 256 different
databases on a single db server.  Any thoughts or advice on that?  Is
that too many databases? not enough?