Just to avoid mistification - the only reason not to use PyQt at the moment is if you have distribution-ready (likely) commercial software that uses Qt under the LGPL license and really don't want pay for a (not that expensive) PyQt license. There is no point stalling python-qt development 'waiting' for PySide to mature - PyQt is a quite complete and release quality binding and switching from one binding to the other is trivial. The development status of PySide should not influence the choice of going GTK or Qt.

PS. Not to mention the far from impossible event of Riverside LGPL-ing PyQt when PySide starts nearing release quality.