Latest kernel (daily build from the 12th) get's me a bit closer:
Instead of booting to a video hang (monitor switch to powersave mode) it now
hangs with the monitor on.

> > Does it hang the entire system as soon as you load the driver, or only when
> > you start X or something like that?
> Hangs on driver load.  
It now hangs on startx.  I can load the system in single user mode and then
switch to dpm=1 fine.  It's when starting X it hangs now.

What is relevant to try now?