Sorry, you're right that the default amazon security policies only allow your nodes to access each other. However, the slideshare I linked to goes to a post about cache _mining_ of EC2 nodes, where open Memcached instances were found on big sites such as Bit.ly, Gowalla, and PBS.
The point I meant to make was: Yes, you can use SGs to make your network secure on EC2, but its very easy to shoot yourself in the foot, which is evident if even larger companies on EC2 are making the mistake. VPC on the other hand provides a secure-unless-you-try-really-hard environment.
NOTE: Modified the post to hopefully make this more clear.
The point I meant to make was: Yes, you can use SGs to make your network secure on EC2, but its very easy to shoot yourself in the foot, which is evident if even larger companies on EC2 are making the mistake. VPC on the other hand provides a secure-unless-you-try-really-hard environment.
NOTE: Modified the post to hopefully make this more clear.