The idea of using OpenGL as a future core rendering architecture for Gtk+ has been brought up a couple times at GUADEC (and then some variations thereof). However there are good reasons to avoid that and major issues with the suggested approaches, in particular these need to be considered:
1) A dependency on OpenGL for a library as portable and as widely used as Gtk+ these days, could only ever be a weak one. That is, OpenGL features may or may not be used, depending on whether the current platform a Gtk+ application runs on actually has OpenGL available or not (e.g. by animating widgets conditionally to only be carried out when acceleration support is present).
2) The OpenGL 2D drawing API is effectively unusable for Gdk/Gtk+ drawing primitives. The main problem here is that OpenGL doesn’t provide pixel-perfect 2D drawing operations which are necessary for accurate input event processing and a coherent visual presentation (it also doesn’t always provide anti-aliasing in the 2D drawing API either). Here is a very good web page with nice screenshots summarizing the problems with OpenGL pixel-perfectness: OpenGL: "not pixel exact", Hardware AntiAliasing.
3) By using XRENDER and hardware-accelerated X drivers, Cairo is already being performance optimized to utilize hardware acceleration. Trying to use a portable OpenGL subset instead (pixel shaders / triangle rendering) would be fairly pointless, it’d effectively be using the available portable hardware acceleration facilities through another indirection. So with more and more Gtk+-based platforms/applications moving to Cairo-based drawing, there is no additional infrastructure or support code needed to make use of available hardware acceleration facilities. Essentially, the portably usable hardware acceleration subset is brought to you automatically through Cairo and X.
For the lazy, here’s a quick overview of the artifacts presented on the OpenGL comparison page: