I was sent a link recently on the Microsoft.com SQL Aerver Architecture, this is really interesting and follows on from a interview I read last year with Jim Gray on channel9 that talked about terraserver using a white box solution rather than clustering.
The key here is that scaling out a clustered solution is very difficult, whereas if your architecture allows for a load balanced DB farm then you can just add new servers. If one fails the rest of the servers can take the load, no server is a single point of failure,
Building such a solution is more complex than a normal architecture, and also requires more manual processes, but is much easier to scale.
I know there has been talk of a (SODA) Service Orientated Database Architecture. This would be best practice guidance for designing, building and supporting a database architecture for different types of services.