Canonical has released an updated series of Ubuntu AMIs for EC2. When starting new EC2 instances, you should use the latest AMI ids to pick up kernel security fixes. If you have Ubuntu 10.04 running on a
t1.micro instance type, you should at least upgrade the software packages to get the patch for the rebooting issue:
sudo apt-get update && sudo apt-get upgrade
For your convenience, the table at the top of Alestic.com automatically displays the latest AMIs published by Canonical, as well as the AMIs that I published for older versions of Ubuntu.