According to qt.nokia.com, Qt is a \"cross-platform application and UI framework\", allowing you to \"write code once to target multiple platforms\". The Qt SDK is a \"complete
I don't think there is a cross-compile configuration.
About the portability of the full project I'd say "almost" because I found a few annoying glitches. None the less your source code will be 100% portable and recompiling on another platform will just require some tricks in the .pro.
What I do is working with just one platform (Linux) and every now and then compiling using my Windows machine and my Mac Mini. What I normally end up fighting with are just a few directives about where to find external libraries or include files. Also when working on Mac I've to copy external files in the app bundle so that the program can find them.