When I updated my oneiric, the update messed up the linux boot altogether.
I have Vista dual boot with Vista primary boot loader.
This time the update left the MBR alone, but grub gave the menu, and every
boot option ended up with "partition not found".
The first time I used alternate install and installed Grub in the beginning of the
root partition. Then the update wrote Grub2 onto the MBR and messed up the Vista boot too.