Tools for prototyping iPad applications (I asked in stackoverflow but repeat it here)

I would like to build a GUI prototype for an iPad application. The prototype can use static data (e.g. an xml file) but should look good and be fully functional, i.e. support user gestures, etc.

Obviously, I can program it in Objective-C. I wonder if I can use any other tool to build such a GUI easier. Does such a tool exist ?

Maybe I should use a GUI builder to build a "static" GUI and add some Objective-C code to make it react on user gestures. Does it make sense ?
Balsamiq: http://balsamiq.com/