I've been having this issue with my hp dv7t laptop. I fixed it using the recommended option: set GRUB_CMDLINE_LINUX_DEFAULT="quiet splash radeon.modeset=0" and run update-grub.
However, that caused a different problem. Before, even though ubuntu would go into a blank screen, the grub menu would always show. Now, GRUB loads (I know that since switching the "beep on load" option works), but I don't see the menu, or anything else, until the timeout is hit and the default option boots. This makes the system workable, but I don't like not having a recovery option and not having a grub menu in general.
There are no other OSs on the system, it's a clean install of Ubuntu Natty 64 bit.
Hitting shift after GRUB loads doesn't help.
How do I get the GRUB menu to show up?