How to implement multi-user websites like Wordpress.com?

I'm going to build a multi-user website, much like wo开发者_高级运维rdpress.com, that users can signup and get their isolated account at a sub-domain.

How this could be implemented efficiently? I'm not asking about assigning sub-domains to users, I want to know about the application architecture which could make this possible.

This application architecture is commonly called "Multitenancy" or Multi-tenant applications where there is a single instance of the application that services multiple/isolated users.

see: http://en.wikipedia.org/wiki/Multitenancy

There are some further links at the bottom of that page.