IronRuby already 'runs' Rails today. We demo'd it running Rails back at RailsConf 08. That said, we have a lot of fit and finish work to do on IronRuby to get it to a state where it can run Rails well.
Where we're lacking right now is:
Startup speed - this is critical for a decent experience when working with Rails - especially if you're running unit tests. Right now it takes 26s to startup Rails.
Working set - we're currently at 100MB for a 'hello world' Rails app vs. 30MB for MRI.
We're actually doing fine on throughput - once we're compiled things run reasonably well.
We're moving towards an adaptive compilation strategy where we interpret by default and compile the hot spots that we find. This will make the developer experience much better. We will also have an ahead-of-time compilation mode (perhaps in conjunction with NGEN and intelligent caching) down the road which may be better than the adaptive compilation strategy.
