The 2.6.22-14-generic kernel on ubuntu seems to have a problem with sata controllers used by Dell. I first experienced this problem when setting up a new kubuntu machine yesterday and today it also showed up on my ubuntu system. Both machines are Dell Vostro 200 (having the same name but for some reasons only dell understands with different cpus).
The boot process takes a real long time and then stops with a busybox prompt and a line

(initramfs)

A short while later message likes the following start to pop up:

[ 60.101981] ata2.01: failed to set xfermode (err_mask=0×4)
[ 95.974137] ata2.01: failed to set xfermode (err_mask=0×4)
[ 131.706301] ata2.01: failed to set xfermode (err_mask=0×4)
[ 136.705051] ata2.00: failed to set xfermode (err_mask=0×40)

At least for this machines the solution was adding the “irqpoll” parameter to the kernel. For a quick start this can be done by pressing ESC on the grub prompt, then pressing “e”, selecting the line beginning with “kernel”, pressing “e” again and adding “irqpoll” to the end of the line. After this press enter and then “b” to boot the kernel.
To make the change permanent the /boot/grub/menu.lst has to be edited. Open it in your favorite editor using sudo (e.g. “sudo vim /boot/grub/menu.lst”) and change the following line

# defoptions=quiet splash

by adding “irqpoll”

# defoptions=quiet splash irqpoll

and then running “sudo update-grub”.