Snow Leopard upgrades a number of scripting languages (including Ruby) to be 64bit which is all well and good until… you try and use a gem with native extensions. Any gems which mention “Building native extensions” during the install process will need to be upgraded. There already exist a number of upgrade guides for databases and the like, but ruby-debug seems to have been forgotten.

To get ruby-debug (and debugging in a rails application) to work:

sudo gem uninstall ruby-debug
sudo gem uninstall linecache
sudo gem install ruby-debug

You’ll have fresh new copies of linecache and ruby-debug ready for you to use on Snow Leopard.