Is there a simple way to expose, say, NSString or UIColor properties of a class such that they can be modified in interface builder?
This would be useful e.g. for adding color properties to a custom view (which are then used programmatically) so that they can be manipulated in Interface Builder as is appropriate.
This article from Cocoa with Love describes how to do what you want with Interface Builder Plug-ins for Mac development. Unfortunately, it is currently not possible to use IBPlugins with the iPhone SDK (note the Interface Builder Plug-in Programming Guide only appears in Mac OS X documentation, not iOS documentation).