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 know about Linux systems. But for developing Qt app targeting Mac OS X and Windows, you need a development machine for each of them. There's no cross-compiling for the two.
Qt is cross-platform in the sense that the same source files can build app for different platforms. With all the VMs available, a one (physical) machine setup to develop and build multiple targets shouldn't be too much of problem. In fact, that's exactly how I am working on my current project which targets both Mac and Windows.