To make a web-based game, would one use something like jruby with javafx or java web start ? Or maybe IronRuby with Silverlight? Or HotRuby with Flash? I'm currently learning Ruby so I can eventually make some sort of web-based game.
I think it kind of depends on what kind of game you want to make. Are you thinking about just using ruby for the backend? Or is your whole game going to be on the client side in the browser, in which case... you should probably go learn flash.
In theory, you (or someone) could create a Ruby library that works with RubyJS to interface with a <canvas> element or SVG to do drawing, etc. It could be called RubygameJS.