Unless you're planning to deploy to a production server running OS X, I don't see why you'd care about Rails running natively on your Mac. Why not develop against a VM running the same OS as your server?
Why on earth should you have to run a VM in order to run a programming language? This hasn't solved the problem of Ruby being difficult to install, it's just added ANOTHER problem of having to learn to install a VM manager and a whole new OS, plus using up far more system resources. Seems to me like that's just making things worse.
It solves the problem of Rails being hard to install on OS X... because it obviates the need to install Rails on OS X. And there's no new OS to learn -- the VM runs the same thing as your server. If you don't know how to use the OS that's running on your production server, that's a bigger problem.
I've been developing this way for years. Additional benefits include: you can easily have multiple VMs with different configurations, you can snapshot/backup/restore a VM very easily, crashing a VM doesn't crash your computer, and it's relatively easy to share a fully configured VM with another developer (even if their primary OS is something totally different).