Roman Hausner wrote:

>> Roman Hausner <roman.hausner / gmail.com> wrote:
>> [.. gui, native, winxp, linux, osx .. ]

Did you have a look at SWT? You would need JRuby to run it, but SWT 
gives you native widgets on OSX (Carbon), Windows and Linux (GTK and Motif).

To start out you may want to have a look here: 
http://www.jvoorhis.org/articles/2006/09/10/hello-jruby

Cheers
Mariano