If I need to access my computer, 

(wait for a while)

Then vgchange -ay (Without sudo)

Press ctrl-d

then login:
tuomas
password

edit: 19.03.2016: I guess I needed to dismiss at some point some message by
pressing enter. Anyway, try to listen to beeps what's going on.

edit: 24.08.2016: With Juha's Nvidia 8800, I am not able to hear beeps on boot
up. Does this now work!? Have I logged in? Did I need to press ctr-c and/or
esc at some point? I can't remember, and I don't know if I'm logged in or not, as I am not
able see anything. Network traffic is not visible either in the eth0. I need a
monitor to make sure what is happening.

Edit: 25.08.2016. Victory! After 598 days, system boots again. Backups resume.

In an event a grub upgrade breaks my lvm + raid stack, (like it did once now, I was
sloppy, see: http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=589737), this is
how I fixed it.

Grab a proper live cd (64-bit on my case) live cd. Any one will do, here's one:
http://www.ubuntu.com/desktop/get-ubuntu/download

Boot using the cd and once done open the terminal.

Ubuntu doesn't have the raid or logical volume management tools out-of the box
installed, so: 
sudo aptitude install mdadm lvm2

In case you don't see your md drives (cat /proc/mdstat), do: 
sudo mdadm --assemble --scan

Now you should see your devices and partitions: 
sudo fdisk -l

And also see your volumegroups: 
sudo vgscan 
sudo vgdisplay 
sudo lvs
...

Activate the volumegroup (if you can't see it in /dev). You'll need the volume
group with the root file system in it, vg_main in my case: 
sudo vgchange -ay vg_main

Mount the root and boot file systems, and in my case also usr and var because
I've made them separate. Lastly bind mount live cd's /dev: 
sudo mount /dev/vg_main/root /mnt 
sudo mount /dev/md0 /mnt/boot 
sudo mount /dev/vg_main/usr /mnt/usr 
sudo mount /dev/vg_main/var /mnt/var 
sudo mount -obind /dev /mnt/dev

Change to the root of your newly mounted system: 
sudo chroot /mnt

Once /mnt is jailed, reinstall Grub to your raid-drives: 
grub-install /dev/sda grub-install /dev/sdb

Hit ctrl-d to exit the chroot, umount the drives and issue 
sudo reboot.

Edit2: 25.4.2011: grub has broken system twice now. Google: "Symbol not found" grub_env_export
This same fix helped.

Edit3: 19.5.2012: And _thrice_ now: http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=662787
Fix was to repartition 1mb with gparted between MBR and sda1, then reissue grub-install: http://forums.fedoraforum.org/showpost.php?p=1543962&postcount=4

Edit4: 15.3.2014: Sure love updating Sid's grub, ("QUARCE!"): http://www.debian-fr.org/sid-initramfs-et-lvm-t47460.html
After aptitude updated grub, I booted into rescue shell, again. My logical volumes were missing: 

ls /dev/mapper/

Should've given: control  vg_main-home  vg_main-root  vg_main-usr  vg_main-var  vg_tmp-tmp

Activating all volumegroups with vgchange -ay and then exiting the shell with
ctrl + d continued the boot. Final fix was to update /etc/default/grub
parameter 

Edit5: 25-09.2014 (entry from /var/log/apt/history.log: initramfs-toolsamd64 updated from 0.116 to 0.117
breaks checkfs. See bugs.debian.org for #763157 and #762870). Resolved by
doing in busybox vgchange -ay)