The OS part is that you're running the Spark firmware to manage comms and security, which talks to the Spark Cloud and makes the rest of it easy. Building the cloud app to be fast, stable, scalable, and secure from scratch is difficult, so this is a SaaS play. Plus, how are you going to manage your 100,000 devices in the field? With Spark you can even do it all with API calls.

and, uh, cough I just joined Spark as well...