I'd really like to do cross-platform. I've ported the rendering core to Direct3D as part of a previous project, so the fundamental pieces are portable to Windows... But there are also some important dependencies on Mac-only components like QTKit and the Cocoa version of WebKit. These would need to be replaced by Windows-native code.

In other words, it's not entirely out of the question on the long term, but I haven't really done the work to even figure out the difficulty.