Either it's a bug or I missed something. It could be that there were fundamental bugs in Ruby 1.9.2.
Again, Rails 4.0 is not out yet, even in betas. We're currently in the planning stages, with a lot of movement on master, so master is in a state of flux at the moment that doesn't really reflect what a release will look like.
Having investigated this a bit, it seems like there are a number of bugs in Ruby 1.9.2 (including some crashers) that make us want to steer people to 1.9.3.
All Rails core team members have running apps on the latest version of Rails, and Ruby 1.9.3 was the first version that everyone felt comfortable shipping to production with. So Rails 4 removes support for Ruby 1.8.7, and requires the first "stable" version of Ruby 1.9, based on our experience bringing apps to production.
How can the Core team claim to support 1.9.2 in Rails 4.0 when the minimum version of Ruby required to run the test suite is 1.9.3?