For some reason, when I emerged gtk+2.4.0 on my laptop, all my theme engines stopped working. I tried re-emerging the theme engines and even restarting X and restarting the laptop and all the theme engines still failed to load. Several of the themes had missing icon problems. I switched back to gtk+2.2.4-r1, recompiled the theme engines which I had recompiled for 2.4.0, and everything worked fine again. My desktop however runs gtk+2.4.0 and all the theme engines are still working perfectly without any problems. I'm not sure exactly what's causing this, but I do know I've had several invalid libtool archives (/usr/lib/something.la) on my laptop which caused certain packages to not compile and I had to copy the appropiate .la files from my desktop gentoo machiene.
I had this same problem. Running 'qpkg -f -nc /usr/lib/gtk-2.0/2.2.0/engines | xargs emerge' as suggested in the ebuild fixed everything.
Thanks. That worked perfectly. Guess I should have checked the ebuild file first.