If I had to do a cross-platform app, I would make a shared core in a language that can run on all platforms[1] and than a native UI in whatever language/toolkit is used for the platform. This way, the only duplication that is needed is the UI. And honestly, cross-platform UI toolkits never quite fit in properly anyway.

1. I'd personally use C. But, thats only because I'm more comfortable in C than any other cross-platform language (C++, Java or any other JVM-based language, etc).