I have the same problem with the latest (5336) drivers in 2.6.2-pre8.
Works fine in 2.4.25-rc?.
It hapens every time I quit the X server. It is frozen so deep that SysRQ keys don't
work and I cannot login from a different computer.
Sometimes I have black screen, sometimes vertical yellow and pink stripes.
I read something that to restore the text mode, the driver needs to access BIOS INT 10. I don't know if can do that just without anything or if it nees a kernel module to be loaded.
Other thing is that I only tested it from vesafb and rivafb framebuffers. I Will try it from a VGA console.