Disclaimer: atm this is mainly for my own exploration of AWS and EC2 - if you're looking to do serious work I would recommend http://star.mit.edu/cluster/
or for an extensive list: https://wiki.python.org/moin/ParallelProcessing
Use boto to communicate with EC2 and thus supports all the same methods of storing credentials that it does. These include declaring environment variables, machine-global configuration files, and per-user configuration files. You can read more about these options on "boto's configuration page":http://code.google.com/p/boto/wiki/BotoConfig.