I'm encountering the same problem as here: http://www.quora.com/Amazon-EC2/Is-it-possible-to-rescue-an-EBS-volume-which-has-marketplace-codes.
Basically I've locked myself out of the instance and I don't have any snapshots. I was working on a larger setup and for security reasons I tried setting up UsePam no in sshd_config which borked everything.
I used the official Debian Amazon Marketplace AMI. Now I'm not allowed to mount the volume somewhere else as a secondary device to fix my stupidity.
The solution listed in the quora thread isn't working. I tried it with two ubuntu images and the instance won't boot with that user data set.
Is there a recent workaround? I'm not trying to access anything secret since I have been using the stock Debian AMIs. I just want to fix that issue without having to re-do everything.