Comment 11 for bug 634102

@Ray,
  It seems to have been fixed for me. I suspect that you're either not running the AMI you think you are, or some process of your own is adding entries to /etc/fstab.

# us-east-1 ami-1cad5275 hvm/ubuntu-natty-11.04-amd64-server-20110426
$ ec2metadata --instance-type
cc1.4xlarge
$ ec2metadata --ami-id
ami-1cad5275
$ grep -v "^#" /etc/fstab
proc /proc proc nodev,noexec,nosuid 0 0
LABEL=uec-rootfs / ext4 defaults 0 0
/dev/xvdb /mnt auto defaults,nobootwait,comment=cloudconfig 0 2

# us-east-1 ami-4d448424 hvm/ubuntu-natty-daily-amd64-server-20110829
$ ec2metadata --ami-id
ami-4d448424
$ ec2metadata --instance-type
cc1.4xlarge
$ grep -v "^#" /etc/fstab
proc /proc proc nodev,noexec,nosuid 0 0
LABEL=cloudimg-rootfs / ext4 defaults 0 0
/dev/xvdb /mnt auto defaults,nobootwait,comment=cloudconfig 0 2

I also verified 'reboot' was functional on cc1.4xlarge both no '--block-device-mapping' arguments and with:
 --block-device-mapping /dev/sdb=ephemeral0 --block-device-mapping /dev/sdc=ephemeral1