One reason that happens is if your root partition is unbootable for some reason.
Try the following:
(1) Boot from the Ubuntu Live CD if possible, or another Linux Live CD and try to mount the root partition on your hard drive manually. See if there are any errors.
sudo mkdir /media/try
sudo mount /dev/sda1 /media/try
(substituting your root partition number for 'sda1'
(2) Try delaying the boot, so that any modules necessary to boot your drive are sure to be loaded before it tries to mount:
(a) wait a few seconds after the busy box prompt appears. Type exit. The boot may continue.
(b) for a more permanant solution, add a bootdelay=[somenumber] to the bootline in grub.
Deails for both of these here
(3) Of course you can also try installing Ubuntu 9.10 instead (released 2 weeks ago), to see if whatever hardware quirk you're experiencing has been found and addressed.