One of the things I'm trying to work out is the best way to approach writing GUI apps with Ruby. There are a number of options including wxRuby, fxRuby, QTRuby, Ruby/tk, RubyCocoa, and so on. Since I'm really interested in an Aqua interface (for my own benefit) I narrowed the field to wxRuby and RubyCocoa. The latter looks especially promising to me although I'm not making much progress on it.
Today I came across CocoaDialog which might act as a neat stop-gap for some of my needs. It's designed to be called from a script that needs to get user input or present information. So in the same way that you can use ruby-growl to display notifications from a script, you can use CocoaDialog to collect input.