Try these steps if you are experiencing a blank screen during boot or install of Ubuntu 11.04 Natty Narwhal.
1. LiveCD boots into a blank screen.
When you see the boot menu when you boot LiveCD, hit F6 and select the “nomodeset” option.
2. Boots into a blank screen after install.
Reboot and keep your finger on the “SHIFT” key till you get the grub menu. Highlight the first entry and replace “quiet splash” with “nomodeset” . Hit CTRL + X to continue booting. Once logged in install the latest Graphics drivers. System -> Administration -> Additional Drivers
DONE. Hope it works for you, it did for me.