This is just a quick note to help me (and others) in case I run into the same problem again. I was setting up Amazon EC2 following This tutorial. When it came time to test my instances:

powerbook-2:~ shannah$ ec2-describe-images -o self -o amazon
Client.AuthFailure: AWS was not able to validate the provided access credentials

The problem was that I had to make the permissions on my private key and cert files to be more restrictive so that only I could read them.
i.e.

chmod 600 chmod 600 pk-7HIOWAY3SOPV7G2LQLMMXEKAH5CKJ5HY.pem
chmod 600 cert-7HIOWAY3SOPV7G2LQLMMXEKAH5CKJ5HY.pem

That fixed it.