Inside Myspace.com - (great info on how they scaled)
baselinemag.com
baselinemag.com
On the other hand, a site should be able to handle several hundred thousand users with a single beefy database server, remote static file hosting, and a few load-balance webservers. When you start 'scaling out' to a bigger array of cheap hardware, you introduce complications that make your site harder to maintain and improve. That change will need to happen at some point along the growth curve, but it seems to me that seed stage entrepreneurs need to be most concerned about making a product that people want to use.
I would have done it a lot different myself :p