I'd been thinking that WordPress might be tricky to scale, but between WP-Cache and the newly announced HyperDB, I think WP might well have some legs in it.
WP-Cache stores pages as static files, and dramatically reduces the load on the database. This makes sites more responsive, and at least theoretically able to survive a Slashdotting or Digging.
Matt just announced the other side of the equation. Enterprise-level database connectivity. They're releasing the (previously custom) database class that was developed for WordPress.com. It obviously works, as WordPress.com has something like 47 quajillion blogs hosted, with pretty decent performance.
Matt's notes on the HyperDB release describe features including:
- Replication
- Failover
- Redundant (public/private) networks
- Local and remote datacenters
- Partitioning
- Different tables on different DBs
- Advanced stats for profiling
- More...?
So, it supports spreading databases across a bunch of servers, making it easier to set up server clusters to scale WordPress (and WPMU) up to any level you want. Might be handy for, oh, I don't know... an institutional blogging platform?
WordPress just got a bunch more interesting, from a CMS perspective...