Comment 5 for bug 649591

As pointed out in Eric Hammond's post, this bug can be fixed in a booted instance with:
 - sudo perl -pi -e 's/(nobootwait),(\S+)/$2,$1/' /etc/fstab
or, if you prefer sed:
 - sudo sed -i 's/\(nobootwait\),\([^[:space:]]\+\)/\2,\1/' /etc/fstab

That will change:
/dev/sda2 /mnt auto defaults,nobootwait,comment=cloudconfig 0 0
to
/dev/sda2 /mnt auto defaults,comment=cloudconfig,nobootwait 0 0

If you haven't yet rebooted the instance, you will not need to take any further action. If you *have* rebooted the instance you will see 'mountall' spinning. That can be fixed with:
sudo stop mountall

Alternatively, you can avoid the bug when you start the instance by launching with user-data like
$ cat ud.txt
#cloud-config
mounts:
 - [ ephemeral0 ]
$ euca-run-instances --user-data-file=ud.txt ...

That indicates to cloud-config that it should not write an entry in /etc/fstab for ephemeral0 (/dev/sda2 on i386 and /dev/sdb on x86_64).