I just created a new cluster with the 2.2 AMI, and after a long while of installing I checked /var/log/boot.log to see what the holdup was, and it was in a seemingly infinite loop doing apt-get update.
Doing this manually revealed the following:
W: GPG error: http://debian.datastax.com stable Release: The following signatures were invalid: KEYEXPIRED 1344722043 KEYEXPIRED 1344722043 KEYEXPIRED 1344722043
