I'm using UML 18.104.22.168 on a 2.6.19-skas3 host, and observing the following when I try booting with a root fs based on FC4/glibc-
2.6-20. UML gets stuck with a 'Warning: unable to open an initial console'. gdb shows that it's because it gets a stream of infinite segfaults. When I boot off another rootfs and try to chroot into the one with FC4, it segfaults out. strace shows that this happens just after libc is ldopened.. at a call to rt_sigprocmask.
So I tried regressing glibc, and now (with glibc-2.6.3) chroot works, but UML still gets stuck at the same place when I try to boot.