For everyone out there upgrading to Fedora Core 8:
If you receive the error java: xcb_xlib.c:50: xcb_xlib_unlock: Assertion `c->xlib.lock' failed. there is a temporary work-around until the bug is resolved (http://bugs.sun.com/bugdatabase/view_bug.do?bug_id=6532373).
The following article solved the problem for me. Just remember to replace the last sed argument in there with the correct path to your JRE install. In my case, I just piped everything from locate libmawt.so to it.