[paulhauggis]

Netsuite has a different url for each new version of their API (and they keep the old one live as well).

This was great for me because my code never broke, which was important because It was running the back-end of an e-commerce site. It gave me more than enough time to upgrade when I wanted bug fixes/features.

[ageektrapped]

Yes! This wasn't what the OP was referring to, though: he had shipping shrink wrap software with APIs in mind. But still, you're totally right and this occurred to me as I was reading the post, for Web APIs: keep versioning the URL for every shipped revision of your API. You'll have to document like crazy, of course. You can even detect stragglers on old versions and help them get on the new version.