When I realized how Mac OS X Dashboard Widgets work, my first question was "does this mean you could build a desktop app in Rails?"
The idea at the time was met with skepticism, but since then there have been three indicators that this is in fact a very genuine future possibility:
Evan Weaver's upcoming RailsConf presentation
Clairify (thanks to Apollo and RubyAMF)
And of course at least one Rails application which, though technically a Web app, seems to be generally run as a desktop app by most of its users:
Way back in the 80s, when computer manufacturers started putting copy protection on their floppy drives, this was seen as a betrayal by geeks, many of whom believed the whole point was being able to customize software to your needs and take it apart for your edification. It could be that their ideal is going to be expressed anyway. If writing desktop apps is easy, people will do it.
Curious to see how this develops. (And, honestly, if it develops.)
By the way, speaking of OS X Dashboard widgets, there's one for RDoc which you can use as-is or point directly at the Rails documentation.