Annoying waste of an hour
If you have an HP 6550b and its using the intel ironlake mobile graphics adaptor, you may find that ubuntu 10.04 and 10.10 don't handle the graphics properly. You'll get a black screen or a failure to start X.
The only way to boot seemed to be to use the VESA driver.
Typical errors in Xorg.0.log are; no kernel modesetting driver detected
into /etc/default/grub (Kernel Command Line) and add to your current kernel boot command line in /boot/grub/grub.cfg.
UPDATE: It seems as though something has changed. I had a similar problem on a far more recent laptop (a sandybridge based dell 6420) and kernel, and it seems that the kernel parameter is now;
So, if one doesn't work, try the other!