Yes, it's about language and a lot more than that
Lorna, Development language is one important issue. For example, Apprenda supports .Net languages and Java; Engine Yard supports PHP, Ruby, Node.js and Java; Google App Engine supports Python and Java. Cloud Foundry supports many open source languages, plus .Net through its Iron Foundry component, but I don't think commercial implementations support Iron Foundry. So yes, languages are an upfront issue. But PaaS is a lot more than that, and developers decisions may rest on what the PaaS can connect them to. Or can it give the application they're building elasticity? How much can it help with deployment? Red Hat OpenShift will let you use Docker; others, they're working on it. It's actually a bigger decision than language and small players may have advantages. Engine Yard and Heroku were early Ruby platforms. Active State's Stackato. like OpenShift, is an early Docker supporter, with continuous integration features.