No breakpoints/single stepping under RDT in Eclipse on Linux?

[wolfram]

Hi,

I have Eclipse 3.1.1 on Ubuntu Linux "Breezy" with Ruby 1.8.2 and RDT
0.8.0 installed.

The manual says something obscure:
"If you can debug your ruby application depends on the version of the
ruby interpreter you are using. Ruby 1.6.8 works on both Linux and
Windows, 1.8.0 and 1.8.2 works also on Windows."

But I don't know how current this is. I didn't find anything obvious
on this list nor elsewhere.

I have no trouble runnig my app in debug mode ("Debug As" -> "Ruby
App"), however, I don't seem to be able to set any breakpoints or do
single stepping--the icons are all disabled.

Is this an issue with RDT on Eclipse on Linux or just a setup problem?
I suspect the latter, as I'm totally new to both Eclipse and RDT. In
either case, help is appreciated.

Thanks,

Wolfram