Well, Bus Scheme has matured just a bit. It's gotten to the point where the gaps in functionality are a bit less crippling. But the more interesting bit is that it's hosted at a publicly accessible repository now, so you can grab it if you're interested.
$ git clone git://git.caboo.se/bus_scheme.git
I've submitted a rubyforge project, and it should get approved soon so I can upload to it. At that point gem installation should be possible with an ordinary, everyday sudo gem install bus-scheme.
At this point there are still some pretty hairy problems with scope; the implementation is as naïeve as possible. Lexically scoped it is not, and closures are a ways off still. Basic λ functionality is a go. You can peek at some tests to see how far along it is. Everything uncommented should pass; commented tests mean "to do" items.
Also, I've tested it out and it works in Ruby 1.8, Ruby 1.9, and Rubinius. A few folks on the Rubinius team have expressed interest in getting it to compile down to Rubinius bytecode, which would be pretty exciting.
REPL it up!
Update: It's now on Github.