Standalone Business Logic Server. Backend business logic servers are designed as pure SOA stateless services exposed via XML over Http. The stateless nature of these services helps scale up or down seamlessly based on load. As business logic is separated from user interface both can be tested independent of each other and the loose coupling also helps them to evolve more independently. A number of solutions are available to scale the business servers. Scalr is one of the earlier ones on Amazon Cloud and now Amazon has an in-built scaling mechanism in its services.
Distributed Database and Storage. RDBMS databases though not designed to leverage distributed computing natively have a surprisingly large number of add-on solutions to make them scale. Solutions vary from having a distributed cache in front of a database to a Master Slave architecture to separate writes and reads. Xeround has a distributed scalable database service and now Google App Engine offers a SQL database service as well designed for scale. Another way to scale transactional data is to embrace document database model where a host of services from CouchDB to MongoDB are available out of the box. Document database model has the added benefit of flexibility in changing the business layer faster but will need throwing away the familiar SQL for a custom API interface to the data layer. Scaling file storage is a lot simpler than the database with services from Amazon S3, Hadoop readily available to use.