QT 4 has been released and it supports Linux, Unix, Windows, and Mac. Anybody who is planning on developing front-ends and other software should heavily consider using it.
It offers cross-platform compatibility.
It is easy to use (by C/C++ standards).
It has dual licenses. You can use the free GPL version, but if halfway through development, you decide that you want to release a commercial app, you can get a commercial license.