Examples of architectures designed to run on Amazon Web Services are a great way to illustrate the necessary design changes and patterns associated with a cloud deployment methodology.
Soocial.com, a "one address book solution to contact management" runs entirely on AWS and uses some interesting technologies to make their service work, including RabbitMQ, an open-source implementation of AMQP, the emerging standard for high-performance enterprise messaging. (I've written about AMQP and RabbitMQ , and in the past.)
One of the most interesting things is how the architecture isn't dramatically different than it would be if you were to build an on-premise version--except that Soocial is able to take advantage the Amazon Web Services EC2's scale, hot-standby, and backups. It's definitely worth learning more about if you are interested in the cloud.