Using IronRuby/IronPython, you should technically be able to do anything you could do in C#/VB .Net. I'm curious if there is anything that would keep a serious rubyist from writing a WPF app for Windows/Silverlight if they were inclined. A post here mentions problems with Silverlight and bindings, but I think there are ways around that.
I'm working on an MVVM library that makes heavy use of dynamic, and I wanted to try it out with some examples using dynamic languages.