When you are building a service it's important to know early on what kind of performance and scalability you can expect to achieve. However. measuring this isn't an easy task and there are lots of variables to take into account - the hardware, the bandwidth, the disparity between production and development platforms, logging while in development, etc.
I've been working on a new project recently using a strict platform model -- interaction between the various pieces is all via restful http services only (so nothing except the api has access to the underlying datastores etc). This means that we should be able to scale out horizontally very easily by throwing some load balancers between the various layers and that we should be able to test each of the interaction layers with a http load testing tool to get some idea of our capacity and performance under load.