Grub2 (Wheezy) booting off either disk in a RAID1 mirror

[Andrew Wood <andrew@perpetualmotion.co.uk>]

Ive got 2 disks in a RAID1 mirror and am trying to setup Grub 2 so that 
I can boot from either disk if one fails.
Ive tried using dd to copy the MBR over but when booting from  one of 
the disks I just get the word GRUB which I guess is because its looking 
for the stage 2 loader on the other disk?

How can I configure Grub2 correctly such that I can boot with either 
disk removed?

Thanks