my machine is IBM ThinkPad T43p [ATI FireGL M24 GL 3154 (PCIE)], but other people report this with different hardware, the issue seems to be the same as reported here: http://lists.freebsd.org/pipermail/freebsd-questions/2003-September/019065.html
...among other places.
what seems to be happening is that card's FIFO gets full and X.org gets stuck calling DRM_IOCTL_RADEON_CP_IDLE that keeps returning EBUSY and never succeeds.
what is interesting, though, is that in 2 out of [many] tries X.org did actually manage to start and worked fine from then on until next reboot. so it seems to be intitialization issue. perhaps a race of some kind?
anyway, i recompiled the kernel with DRM_DEBUG, enabled RADEON_FIFO_DEBUG in radeon_cp.c and got some debug info which may be useful. complete Xorg.0.log is also available.
the visible effect is that the video mode changes, but the screen stays blank.
machine is responsive, sshd works, but switching virtual consoles is not possible.
i am very much willing to assist with applying patches and providing more info.
Closing due to inactivity. Please reopen if this is still a problem.