Though interestingly, he doesn't really mention 301 redirects. Is the point that URLs shouldn't change, or that changing URLs shouldn't break the web (or a little of both)?

Considering that no URL shortener I know of will actually let you change the underlying URL it points to, unless the hosting service decides to change what the underlying URL actually points to (tsk tsk), the shortener is pretty much a canonical pointer to that data, despite the fact that the hash doesn't often give much information about the underlying data.

Also: see http://www.301works.org