I think Dropbox is considered SaaS (and doesn't precisely fit your definition of "web application"). I would put S3 into PaaS as it boils down to a storage platform.
Right now I agree with saying IaaS is just a VM service, but I'd hesitate because there may be future changes that invalidate that connection.
Keep in mind these terms are provider-centric not customer-centric. For example, Amazon provides their networking and computer infrastructure as a service to you the customer. You realize this service as VMs. Similarly, GAE provides a platform for you to use, and you realize this as a place to run your cloud application. And to finish the triad off, SaaS provides customers with software and they realize it as an application. So it somewhat depends on if you are looking at these things from a customer perspective or a provider perspective.
Right now I agree with saying IaaS is just a VM service, but I'd hesitate because there may be future changes that invalidate that connection.
Keep in mind these terms are provider-centric not customer-centric. For example, Amazon provides their networking and computer infrastructure as a service to you the customer. You realize this service as VMs. Similarly, GAE provides a platform for you to use, and you realize this as a place to run your cloud application. And to finish the triad off, SaaS provides customers with software and they realize it as an application. So it somewhat depends on if you are looking at these things from a customer perspective or a provider perspective.
I believe this NIST document provides fairly clear terminology: http://www.nist.gov/manuscript-publication-search.cfm?pub_id...