If you don't like that (for example, you don't want to distribute your code as mandated by the GPL but don't want to pay for QT), another good platform-independent GUI toolkit is wxPython:
Seconded. The QT licensing is actually more restrictive than other dual-licensing schemes: TrollTech will not allow you to start developing your application with a free version (including the ones distributed with Linux), then switch to a commercial license later. In other words, if you think there is any chance you'd want to distribute any of your application's code under a non-GPL license, you need to get a QT commercial license before a line of code is written.
PyQt all the way! Both for my hired gun projects (WengoPhone, QuteCom) and for my personal projects. The UI toolkit (QtGui) is great: the widgets have a native look and feel on all platforms (deal breaker otherwise for me), you have an good UI designer etc. But Qt has more libs! File parsers, a signal/slot library for asynchronous calls (which you pretty much have to use with QtGui).
