I have been converting (on my development system, not in production yet) my Rails applications to work with Ruby 1.9.1. I converted my non-Rails stuff (that I care about) to work with 1.9.x a while ago.
I still use ferret for single process (or single thread) web applications. I have wanted to switch to Sphinx and UltraSphinx for a while and since ferret is not compatible with Ruby 1.9.x because of changes for unicode (RString struct, specifically) I did that conversion late last night. I was pleasantly surprised how simple it was to switch to Sphinx and UltraSphinx: lots of little edits, but quick and easy.
A little more frustrating: the auto_complete Rails plugin does not seem to be Ruby 1.9.x compatible in one of my Rails web apps. I am going through the plugin code, adding debug output, and looking for the problem. The problem may just be specific to my application.
Because of the performance improvements of 1.9.x and because of the long term maintenance overhead of having (eventually) both ruby 1.8.x and ruby 1.9.x applications in deployment, I am willing to spend a lot of effort on the conversion. Another issue is that I expect to have a Ruby book published early summer of 2009 and I would like all the examples to be Ruby 1.9 compatible (and some JRuby specific code).