Tim O'Reilly cites Ian Wilkes in Web 2.0 and Databases Part 1: Second Life:

"Like everybody else, we started with One Database All Hail The Central Database, and have subsequently been forced into clustering. However, we've eschewed any of the general purpose cluster technologies (mysql cluster, various replication schemes) in favor of explicit data partitioning. So, we still have a central db that keeps track of where to find what data (per-user, for instance), and N additional dbs that do the heavy lifting. Our feeling is that this is ultimately far more scalable than black-box clustering."

Follow-ups: Database War Stories #2: bloglines and memeorandum, #3: Flickr, #4: NASA World Wind.