The lead developer of KWin has shared an update regarding the work he and others have been doing to bring KWin to Qt5 and KDE Frameworks 5, for the eventual release of KDE 5.
For making the transition, they've had to change their X Server interfacing from Xlib to using the modern XCB, changing around QGraphicsView code to using Qt Quick, porting more code to using QML, and improving the X RENDER compositor. With the X Render compositor improvements, also ended up bringing improvements to the OpenGL compositor due to re-thinking their rendering code paths.
In terms of where they are at in their transition to Qt 5, a test compile last week with Qt 5 and KDE Frameworks 5 revealed a few issues but KWin did successfully build. There's still some low-level issues like needing some of Qt 4's native graphics system and a few issues within KDE Frameworks 5.
Martin's focus for working on this porting right now is for testing their event filter from Xlib to XCB and they can only test that in a Qt 5 environment. The final transition to Qt 5 and KDE Frameworks 5 will come later.
Find more details within Martin's blog post.