Running PrimaryBlogger is a lot more complex than running one or two wordpress blogs.
We use round robin DNS for high availability.
We use Varnish Cache to speed up pages, load balance and stop Wordress having to work so hard.
We use MySql replication to replicate database entries.
We have a proprietary system that we use to replicate the file system (images etc.).
We use Apache as our web daemon. We use WordPress, obviously..