That is a novell idea. It is like balsamiq, but low tech. They should sell it to every ad agency. Because the one were I work, we continually get asked about making iPhone apps.
I hope they design one that is made for website ui as well.
Looks like you can buy it from their shop if you'd like it.
This kits looks biased towards the "selection list" UI metaphor similar to how iTunes works. It doesn't seem useful for any apps that would involve any degree of graphics or visualization. (And of course, games are right out.)