If I were them, I'd look at an approach that was based on either SimpleDB or S3. I've been working on a couple of prototypes of systems similar to theirs for my own stuff at work, and I've been toying with a system that uses S3 to store what I'm calling "absolute fallback" sources of data for conversations (basically, JSON documents) and SimpleDB as a front-line store.
My general take on this issue is that if you're running your app on EC2 and your persistence medium is something that's also on EC2, you really have no ideal high availability scenario. Of course, even in my case, if SimpleDB and S3 go down, I'm still in trouble, but at least I have the option of throwing Akamai in front of it.
My general take on this issue is that if you're running your app on EC2 and your persistence medium is something that's also on EC2, you really have no ideal high availability scenario. Of course, even in my case, if SimpleDB and S3 go down, I'm still in trouble, but at least I have the option of throwing Akamai in front of it.