If you had or have a black screen or purple screen problem in Natty, please be counted, add support by subscribing to this Launch Pad Bug:
Xorg's HAL causes Purple Screen or Black Screen on GDM startup.
Through helping people with these problems in the Ubuntu Support Forum (Installations and Upgrades) and through this sticky:
...this bug (above) seems to be where this problem is originating. It affects this problem with all video hardware types.
As I said in this bug report:
"Normal users should not have to be highly skilled "techies" to manually bypass a broken part of a system to make a released distribution work during an install or distribution upgrade"