For debugging ruby stuff with komodo the ruby.debug library is necessary.
Unfortunately the version which is provided by komodo itself is only a 32-Bit version.
But MacOS 10.6 is now mainly a 64-bit operating system.

Solution:
* rename or delete the following directory:
/Applications/Komodo IDE.app/Contents/SharedSupport/dbgp/rubylib/1.8/

* make sure you have installed the XCode Extension for Snow Leopard and downloaded the latets update for it

* install the current version of ruby-debug-base via gem:
sudo gem install ruby-debug-base

This one builds a native extension for 10.6, thats why XCode is necessary. Otherwise gem will complain on missing headers etc: e.g.
/System/Library/Frameworks/Ruby.framework/Versions/1.8/usr/bin/ruby extconf.rb
mkmf.rb can’t find header files for ruby at /System/Library/Frameworks/Ruby.framework/Versions/1.8/usr/lib/ruby/ruby.h