Forrester has done quite a number of reports in the last two years around platform-as-a-service (PaaS) from the long-term strategy perspective from me and from the application developer perspective from my friend John R. Rymer. During this time, we saw many different business cases around PaaS. We have predicted and quantified that the major buying power of PaaS will come out of three camps:
ISVs are buying PaaS technology. This is a model that we saw with many ISVs on major platforms that managed to create a viable marketplace such as salesforce.com's AppExchange and Google's marketplace.
Corporate application developers are using PaaS to deploy custom apps and add-ons around SaaS applications. They are doing this significantly faster and at a lower TCO than before.