[G_Morgan]

If you are looking at cross platform then be careful about what libraries are available. Wxwidgets is probably the best cross platform option for Python but it has it's issues.

Personally I might look towards something like JRuby so you can utilise the Java API. Swing isn't perfect but would be safer than Wxwidgets IMHO (of course you could also use SWT with anything that runs on the JVM which is similar to Wx but better).

There is a Python interpreter for the JVM (Jython) but I picked JRuby because Sun Microsystems have put a huge effort into making it work well with Java.