Comment 70 for bug 1003309

I was struggling on the same problem (Ubuntu 14.04 + software raid1 + dm-crypt) for a while.
The computer started ok with two disks but failed with the "waiting for encrypted source device" message with just one disk.
Note : I have to point that the system is able to start with mdadm, one disk but no encryption.

I finally found the solution here :

It was for a problem with mdadm, but it works beautifully with mdadm + dm-crypt in 14.04 version (And probably until 12.10 version)

In summary, and if you have two software raid partitions (Say /dev/md0 and /dev/md1) :
Create a new file /etc/initramfs-tools/scripts/local-top/workaround_mdadm :
sleep 6
mdadm --stop /dev/md1
mdadm --stop /dev/md0
sleep 6
mdadm --assemble --scan

Make the file executable :
chmod 755 /etc/initramfs-tools/scripts/local-top/workaround_mdadm

Create new initrd files in /boot :
update-initramfs -k all -c

Reboot with just one disk and enjoy.

The problem seems related to the fact that /etc/initramfs-tools/conf.d/mdadm with BOOT_DEGRADED=true doesn't exist anymore for the 14.04 version. If you create it, the problem still exists.
With this workaround, you will be able to obtain the passphrase asking and boot with no problem.