I was just toying with such an idea and wondering why that wasn't thought before. Use webkit as the base GUI widget tied with Python/Ruby/your_favorite_lang and you get:

* Truly cross-platform GUI on major OSes.

* Smoother transition if your app needs to move on web.

* Pretty looking UI's. 5 years ago, I'd prefer VB/Delphi/.NET [1] for building state of the art UI's (backed by some commercial packages) but today the scene has been completely changed. Hard to catch up with JQuery and alike when it comes to good looking UI's with the tools above.

Downsides? Having a fully blown JS interpreter working aside and a few megabytes bigger installations? Well, I'd say, way to go boy!

Glad to see someone actually working on this idea. I hope they don't make the mistake of setting a high price for entry which would unavoidably push it into being another niche framework. Best of the luck!

[1] and good luck on Linux!