The latest high-profile open-source project jumping ship from the GTK+ tool-kit and migrating to Qt is the Wireshark network analyzer.
Wireshark has been around for a long time and has always been a GTK+ application, but with their increasing multi-platform focus and looking at new targets to support, GTK+ is simply getting in the way. GTK+ on OS X is a second-class citizen compared to other platforms and Qt supports significantly more operating systems, especially for the emerging mobile platforms.
As a result, with this week's release of the Wireshark 1.11.0 development release, the move is underway in ridding the GTK+ code and replacing it with Qt5 code.
More extensive information on Wireshark's migration from GTK+ to Qt can be found via this project's blog post
by Ethereal/Wireshark founder Gerald Combs.