Two Quick Observations
Several of the speakers here have echoed two themes about databases.
1. MySQL is in production in a lot of places. I think the high cost of commercial databases (read: Oracle) leads to a kind of budgetechture that concentrates all data in a single massive database. If you remove that cost from the equation, the idea of either functionally partitioning your data stores or creating multiple shards becomes much more palatable.
2. By far the most common database cluster structure has one write master with many read masters. Ian Flint spoke to us about the architectures behind Yahoo Groups and Yahoo Bix. Bix has 30 MySQL read servers and just one write master. Dan Pritchett from eBay had a similar ratio. (His might have been 10:1 rather than 30:1.) In a commerce site, where 98% of the traffic is browsing and only 2% is buying, a read-pooled cluster makes a lot of sense.