Just a notice for others like me who are using Rails on Debian/unstable. The Ruby packages have been updated to version 1.8.5. While this is a good thing in general, it might be inconvenient to Rails developers as this version of Ruby breaks breakpointer.
A workaround could be this
http://eigenclass.org/hiki.rb?call_stack+0.1.0
I haven't tried it yet myself.
Michael