When you launch an instance in EC2, be it EBS root or instance-store, you are "entitled" to some amount of ephemeral storage. Ephemeral storage is basically just extra local disk that lives in the host machine where your instance will run. How much instance-store disk you are entitled to is described with the instance size descriptions.
The described amount of ephemeral storage is allocated to all instance-store instances. However, at register time of EBS-root instances the default "block-device-mapping" is set. Because an EBS-root AMI may be run with different instance-types, it is impossible to set the correct mapping for all scenarios. For the Ubuntu images, the default mapping only includes ephemeral0, which is present in all but t1.micro sizes.
So, if you want to "get all that you're paying for", you'll have to launch instances with different '--block-device-mapping' arguments to ec2-run-instances (or euca-run-instances). I've written a little utility named bdmapping to make that easier, and hold the knowledge.
You can use it to show the mappings for a given instance type:
$ ./bdmapping
--block-device-mapping=sdb=ephemeral0 --block-device-mapping=sdc=ephemeral1
Or, use it with command substitution when you launch an instance:
$ ec2-run-instances $(bdmapping -c m1.xlarge) --key mykey ami-c692ec94
#
# the above is the same as running:
# ec2-run-instances --block-device-mapping=sdb=ephemeral0 \
# --block-device-mapping=sdc=ephemeral1 --instance-type=m1.large \
# --key mykey ami-c692ec94
You can view or download 'bdmapping' at https://gist.github.com/809587
