That looks really cool, but it doesn't work in Xcode 4.3, unfortunately. I found a bit more information here: http://blog.ablepear.com/2012/02/something-wonderful-new-obj...

Apparently it's a feature in Xcode 4.4 in the beta release of the Mountain Lion SDK. There's no developer preview for Lion, though. Fingers crossed that Xcode 4.4 will be released for Lion and not just for Mountain Lion....