This is pretty sweet!. DTrace support was unofficially there for sometime now (or you could always use FBT), but this makes it much easier for people who do not want to patch/compile.
That commit is for YARV, not MRI
And didn't Leopard's builtin version of MRI have dtrace support?
It was my (perhaps incorrect) understanding that at some point int he 1.9.1 timeline YARV was merged into MRI and became the the MRI trunk?
Nope. Ruby 1.8.x is basically in only maintenance mode. The latest version of MRI was released in May 2008 (excluding minor patchlevels).