Copied directly from jQuery 1.6 Roadmap:

I noticed that undelegate doesn't seem to work with a custom namespaced event.

If you undelegate('selector', '.namespace') it doesn't work, but if you unbind('.namespace') that same thing, it does.

Here's a fiddle that explains what I expected

http://jsfiddle.net/matthewlein/7LRmR/

seems like something useful for plugins or widgets that want to delegate.