There is already a bug filed against xlibs/xcb*-0.3.8, and 0.3.6 has already been removed from portage (a very bad decision).
The major problem is with libstartup-notification, which relies on a function defined in xcb-util-0.3.6 and no longer exists in 0.3.8. I'm going to try to recreate the ebuilds for 0.3.6, but meanwhile *don't* allow xcb* to update to 0.3.8 if you depend on libstartup-notification.
