So far the crawler's only made it back into March or so of 2009, so
unless you are one of a very small group of test users, you won't see
any 404's before then. It will probably take another week or two
before the crawlers can clear the backlog.
There's backend support for all error codes, but I've only exposed 404
for the time being, to avoid false alarms. There are a bunch of
sites that give transient 500 errors, for example, or just don't like
being spidered. Once I have enough confidence that the code
reported by a crawler will be the same thing a user sees in the
browser, I'll make pages for all error types available.