Hmm, even i had such a problem on my first install of 9.04. But after
i reinstalled (not for that reason), it seems to be working well. I
used to switch off directly after it hanged up on the blank screen,
didnt bother to find a solution though. It is a bug i guess. Check
this bug if its the same problem as yours,
https://bugs.launchpad.net/ubuntu/+source/linux/+bug/71004.
nir