Jurriaan Persyn is a Lead Web Developer at Netlog, a social portal site that gets 50 million unique visitors and 5+ billion page views per month. In this paper Jurriaan goes into a lot of excellent nuts and bolts details about how they used sharding to scale their system. If you are pondering sharding as a solution to your scaling problems you'll want to read this paper. As the paper is quite well organized there's no reason to write a summary, but I especially liked this part from the conclusion:
If you can do with simpler solutions (better hardware, more hardware, server tweaking and tuning, vertical partitioning, sql query optimization, ...) that require less development cost, why invest lots of effort in sharding? On the other hand, when your visitor statistics really start blowing through the roof, it is a good direction to go. After all, it worked for us.