I am developing a SaaS App, and I want to decide for a PaaS Vendor. Since one of my biggest concerns is uptime, is there an application or service, that allows me to use several PaaS Vendors (like Azure, Google App Engine, Amazon Web Services, etc.)?

I want my application to be able to respond from one PaaS Vendor to another almost instantly without any downtime, to use the redundancy that this provides. This means that I need to be able to use the different services homogeneously.