In his blog DevOps is DOA Tom Mornini writes that DevOps is already outdated in the face of PaaS offerings. His opinion is not entirely surprising as he is CTO and co-founder at EngineYard, a large PaaS vendor.
Interstingly most people seem to disagree with his opinion (for example here, here and all of the comments at Tom's blog). I also believe that he missed the main advantage of DevOps which is IMHO that DevOps actually supports knowledge-driven teams while PaaS requires one to trust in the service quality of the PaaS vendor with little to no way of influencing that.
So, if you don't want to increase the amount of knowlegde in your organization then PaaS is a good way to go and then you maybe really don't need a DevOps approach. Just wait a while and find some of your developers turned into ops because in the end even in a PaaS world somebody needs to maintain the automation that is used to feed the PaaS hosting.
But, if you want to increase the total amount of knowledge in your organization and if you want to enable your teams to take more responsability for what they are doing and for what comes out in the end then please do look into DevOps as a way to share responsability between all tech teams involved in your enterprise. And if your tech teams decide together to use PaaS then this also might be a good thing because they decided it together.