I recently learned that Jetbrains the make of my favorite IDE (Idea) may support Objective-C (though it is unclear how much it will work for iPhone/iPad development). See the thread here for early discussion on this.
In the last year or two, they have started adding additional language support both in their flagship IDE as well as specialized IDEs (for Ruby, Python, PHP). I guess this is just another step in the process. I for one would love to have another option other than XCode and I couldn't think of one that I'd love more.
This is obviously vaporware at the moment, but I think it is something to keep an eye on.
This is now a real product, albeit still in Early Access. See here for a the blog on this new product, which will give you pointers to check out the EAP.
UPDATE: AppCode has now been released and offers a true alternative to using Xcode for Objective-C and iPhone/iPad/Mac development. It does still rely on Interface Builder for layout and wiring of GUI components and uses the iOS simulator, but all coding, including a slew of refactorings, smart templating and static analysis, is available through App Code.
