The Amazon AWS blog has a good story about about Soocial.com. Soocial.com is an address book on steroid. It works with (or will work with soon): outlook, blackberry, gmail, mac address book and other phones. They just need to add Yahoo and Windows Contacts.
I wrote about it on my cloud blog, 100% AWS.
The AWS blog post has a good overview of the overall architecture. They are using Postgres and pgpool II on EC2 and EBS. They use RabbitMQ for messaging. For high availability they are using HAProxy and memcache.
It sounds like a pretty sweet setup.