I haven't been able to find out what AMI runs on Amazon's elastic mapreduce instances. You can't specify one, but can customize the mapreduce instances by running a boostrap action. However, I'd like to know what I'm starting with.
Currently one is:
It appears that the specific AMI changes over time (and certainly between regions).
The answer can be found at any time or region by:
It also appears that it is possible to launch these into your own EC2 or VPC. I have verified that (at least with the image listed above), you can SSH into it after launching into EC2. I have no idea what will happen to hadoop if you do this or if this access will be discontinued at some point.
According to the FAQ. it's running Debian Lenny. I couldn't find the actual AMI.