I don't know how different wordpress.com is to 'typical' WPMU (wordpress multi-user) as is deployed by e.g. some large universities, where tens of thousands of

I don't know how different wordpress.com is to 'typical' WPMU (wordpress multi-user) as is deployed by e.g. some large universities, where tens of thousands of blogs can exist in one single database instance, served by one single front end which has a wildcard/domain mapper. But I imagine it is not exactly the same. They have tens of millions of tenants AFAIK.