I think this will be a great product for startups -- everyone wants to build products on top of a dynamic environment, but to really do it, you would need to invest in heavy use of things like using Zookeeper -- but Zookeeper has a very high cost to integrate and maintain.

Netflix has released something very similar, Eureka, as open source here:

http://techblog.netflix.com/2012/09/eureka.html

But the Rackspace Service Registry comes with a SaaS model, more client libraries, and is hopefully easier to integrate.

(disclaimer, I work at Rackspace, but not on this product)