Over at F5's Dev Central, Lori MacVittie talks about load balancing and the cloud:

When you decide to deploy an application to the cloud you have a very similar decision: do you extend your existing dwelling, essentially integrating the environment with your own or do you maintain a separate building that is still "on the premises" but not connected in any way except that it's accessible via your shared driveway.

Using a garage expansion as a metaphor for scaling laterally to the cloud is a great one, and captures a lot of the nuances to be dealt with.

I'd like to add a third option to Lori's first two, based on our experience with hundreds of enterprises -- the valet strategy. Rather than simply load balancing between multiple systems, a cloud can sit in front of multiple sites, performing application delivery functions, as well as load balancing betwixt the backend sites.

Many Akamai customers do this today. They may have several data centers of their own, and want to integrate cloud-based services seamlessly into their existing sites (like taking advantage of user prioritization to load balance between an application server and a cloud-based waiting room; or using storage in the cloud to provide failover services). Akamai's cloud accepts the end user request, and either takes care of the user locally, or gathers the necessary resources from among multiple backend systems to service the user. And that's a way to load balance transparently to the user.