I observed several symptoms:
(1) On my login screen (xdm) I run an xclock. Upon login, this xclock gets killed using xkill. Before killing the xclock, all is normal. Once the xclock window gets killed using xkill, the application sticks around and spins one core at 100%.
(2) My .xinitrc starts an xscreensaver session. About half the time, xscreensaver immediately starts spinning one of the cores at 100%.
The commonality I observed between these two symptoms is that in both cases, it is an application that currently doesn't have a mapped window that is spinning the CPU.
My system is an up-to-date ArchLinux system with kernel 3.2.9, latest stable xorg (unpatched). I observed this behaviour both on my desktop system running an NVIDIA NVS 450 and on my laptop with built-in Core i7 graphics. My logs don't show anything unusual (unless I don't know what to look for).
I'm able to reproduce this.
Reverting commit 70bb9e28d37fff4ead4d0a2c672af77059765fa4 in libxt fixes this. Apparently, this is the same bug as already reported as ID 47203.
Reverted with http://cgit.freedesktop.org/xorg/lib/libXt/commit/?id=75bef5b488245467b6638e25cd22006b306e8536