Wow, that looks great Dave.
This is one link for those interested in what this is all about, the link in my notes section in the project is broken, but this looks like the same page.http://lodev.org/cgtutor/raycasting.html
And for what it's worth, you actually could hook this up to an OpenGL renderer if you wanted to, my guess would be it would be a lot of work. Ray Casting is just giving you a fake 3D world, deciding what the pixel color location should be, which is currently passed to the Rb graphics object. You could set up an ortho projection and give OpenGL the same pixel color that is used now, i.e. replacing g.drawline ..... You could also pass the color and location directly to Quartz, CoreGraphics. Or any other renderer for that matter.
Spinning this off to a thread might speed things up a bit too. And refactoring to coincide with the suggestions for drawing with Cocoa would probably help too. Cocoa aside, this runs on Windows and Linux too, right?