I just finished porting the ruby-zoom gem to 1.9.2. Previously, it was only compatible with ruby 1.8.7. So, now you can search library catalogs using z39.50 in your 1.9.2 apps. You can find the repository on Github: ruby-zoom.
This was born out of a need in my Gift Manager app. I don't really know the details of how z39.50 works and full credit is due to the original authors and the makers of YAZ.