I would likely build a front-end using a couple HAProxy load balancers hitting an Apache cluster running opencluster. Use red-black trees with mySQL and cluster a few databases across multiple locations. I would build the front-end with Python and html5, as well as using iphython for cluster controls and other fun stuff.
In my case I have a rack of HP p-class blade servers that use an Amazon EC2 Centos box to route inside/outside of EC2. When we test something out we use my cluster at home, then when we roll an app or website out we keep it at my house. If the load gets high, then we simply modify the cluster to bring up slave web servers, cache servers, etc. In our case we build the backend first and can roll out an app or web service for very little money or resources, but if we have success with something we just leave it on EC2 since it can likely pay its own bills.