Comment 41 for bug 103177

In the grub menu, select the kernel you want to boot with and then press 'e'. Then select the kernel line and press 'e'. Add a space and 'break=mount' to the end of the line and press enter. Press b to boot. You'll get thrown to busybox. Wait a few seconds, and then press ctrl+d.

Alternatively, the way to update the kernel would be to find an old live cd that you know works with the array (I used an old Dapper cd). Then mount your root partition to a directory with a command similar to 'sudo mount /dev/md0 /mnt'. Then 'sudo chroot /mnt'. Your shell will now behave as if your mounted partition is now the root directory until you type exit, so you can wget the new initramfs-tools or the new kernel package or whatever you need and dpkg -i it.