We do code reviews to make sure VIP WordPresses are as fast and stable as possible, but more of that information could be documented. You're right and it's something we're actively working on.

There is going to be a dedicated VIP portal with documentation of all the special functions, lots of theme and system info, and best practice coding guidelines for WP.com.

The cool thing about VIP is you can get, say, a direct link from the Yahoo home page and it doesn't break a sweat. It's specialized to just WordPress, but more people every day are running their entire site through WP.