I know several pieces of production code that are still on Ruby 1.8.x. 1.9.x was adopted fairly quickly in some very vocal subsets of the Ruby community, but despite the fact that the incompatibilities are fairly small there are still people that haven't made the migration.

Personally I still run 1.8.x several places too, not because I have any particular reason to, but because I haven't had any particular reason to upgrade - not a single gem I depend on for my hobby projects require 1.9.x.