For Qt 4.5, there is a IDE named "Qt Creator". For Ruby 1.8.x, there are many IDEs. But for Ruby 1.9.1, IDEs that support 1.8.x don't support yet. Please see InfoQ's article for more information.
But, except syntax of Ruby 1.9.1, IDEs work almost fine. So I planned to use one of ruby IDE, NetBeans. I choose ruby download bundle, neither all-in-one bundle and JavaSE bundle and so on. I download, install, and launch this IDE.
If LD_LIBRARY_PATH must be edited, see also #3, and launch from command line with edited LD_LIBRARY_PATH.
After adding Ruby 1.9.1 platform in menu , I write one sample program from Rapid GUI Development with QtRuby, and run on the IDE. This result is below: