My colleague Joe Hofstader just got an article published on MSDN.  Here's a summary:

Being an architect on a CaaS solution over the last few years has brought some interesting perspective into the unique challenges of the development and deployment of these applications. As I mention throughout this article, CaaS applications require additional considerations past hosted multi-tenant solution development, whose design challenges tend to reside in the orchestration layer and the security services. The SIP services layer requires knowledge unique to communications solutions, and the lack of an understanding of the patterns required to implement these solutions can hinder the effort.

Take a look at Communications as a Service