Comment 1 for bug 413815

Sorry, I didn't explain myself clearly in the initial bug report.
The ruby 1.9.1 package made available in the karmic repository should have been built with --program-suffix=1.9.1 if the binary is going to be installed as '/usr/bin/ruby1.9.1' but it fails to do so.

Possible problems could be(but not limited to) the failure of rubygems to install properly.
Rubygems finds the ruby binary name from rbconfig.rb, and rbconfig.rb will point to '/usr/bin/ruby' if the --program-suffix hasn't been specified.