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.