This happens to me on i686 / sid. It also happened with the wheezy version.
I tried compiling gnome-settings-daemon for debugging and running it with --no-daemon --sync under gdb with a breakpoint set at gdk_x_error but for some reason it did not actually stop for a backtrace when it crashed. It seems much harder to get gnome-settings-daemon to crash when compiled and running in this way :-) and so I had to wait a couple of days. I'll let you know if I find any more info.
