You are still stuck using the GUI toolkits of the platforms in their native tongues. I think Qt and Mono (so C++ and C# respectively) are the only toolkits that support both of them with one code base.
Perhaps not so much a Serious UI Toolkit(TM), but anything that eventually compiles down to GL would work too. This is how play-clj does it: binding to libgdx, which eventually calls some OpenGL methods.
While I did jokingly say "Serious UI Toolkit", there's more than a few awesome applications on Android that have very, very simple UI widget requirements. Particularly games, which according to the article, they're focusing on.
