This post has been moved into the DataStax Documentation pages. A newer and up to date version of these instructions can be found here.
Update (4-20-2012):
The new DataStax Community AMI can be found here.
The new DataStax Enterprise AMI can be found here.
I created a small github repo that will help automate this if anyone is interested:
https://github.com/fuentesjr/cassies-party