While the latest AMD Catalyst driver fully supports up to OpenGL 4, within the KWin compositing window manager for the KDE desktop it defaults to using the OpenGL 1.x renderer. The GL1 renderer is used with the AMD binary blob since using the newer OpenGL 2.x renderer is troublesome for Catalyst. Meanwhile for the NVIDIA binary driver and even the open-source Mesa/Gallium3D drivers, the GL2 renderer works without fault.
Martin's looking at dropping the GL1 renderer since it's just used by Catalyst and old graphics processors (those that are nearly a decade old) so it's not worth the maintenance costs. Dropping the GL1 renderer right now would mean those Catalyst driver users with even the latest Radeon HD 6000/7000 series hardware would be faced by a challenging experience on the KDE desktop.
Fortunately, Martin's unlikely to drop this old renderer for a few months and AMD will be providing proper KWin support at some point going forward (i.e. the proper GL/GLES renderer with direct rendering should be bug-free or at least with minimal issues).
In this ATI bug entry, an AMD engineer mentions, "We are working on the issue and will support direct render for kwin." But when AMD will end up releasing a driver that properly supports KWin is another question... Hopefully it will be by later this calendar year so there will be no fallout for KDE users.
Thanks to "markg85" for mentioning this BugZilla entry.