Brian Donovan shows the basics of using ruby-debug. This is very nice addition to my already outdated tutorial. He also announced that his next screencast will cover debugging of a Rails application.
One the same note, the next version of Rails comes with the native ruby-debug support. The relevant changeset is here.
I'm wondering, what happened to ~/.rdebugrc ? Did it go away? It was my favorite, simple feature of rdebug.