BlueMix was one of the most talked about topics at IBM Impact this year. In a nutshell, BlueMix is IBM's platform as a service built on Open Cloud Architecture and Cloud Foundry, with the intent of simplifying many of the commonly repeated tasks facing developers on a regular basis.
BlueMix provides the building blocks to quickly deploy things like commerce solutions, reduce application and infrastructure provisioning, as well as scale with compute and storage as needed for the application. As BlueMix was being described to me by several different people at IBM Impact, I couldn't help but think of it as being something akin to Heroku.
You can find out more about IBM BlueMix in this interview with Stephen Kinder from IBM Impact.