This bit me: “Unknown class<myclass> in Interface Builder file” error at runtime</myclass>

http://stackoverflow.com/questions/1725881/unknown-class-myclass-in-interface-builder-file-error-at-runtime/5546554

I was having problems with XCode referring to a View Controller I had just deleted and it turned out the linker was loading stuff from cache.

I found this stack overflow question and deleting the contents of this folder solved the issue:

~/Library/Application Support/iPhone Simulator/4.3.2/Applications

Now I can continue learning :D