Dan Kusnetzky's post about OpenStack equates the project with an Open Systems movement. Fortunately, Open Source has nothing to do with what essentially is a marketing term that was mostly coined during the 80's.
Back then, the Open Systems movement was nothing more than promises of interoprability from proprietary vendors. As Dan explains further, these promises went mostly unfullfilled. Essentially it was the birth of the "oldest trick in the proprietary vendor book", where a black box is made more attractive because it allows interfacing via APIs, documentation, and/or standards.
Open Source on the other hand is fundamentally different. There are no promises of interoperability, instead users are given powers far greater. Beyond Open Systems, Open Source delivers ultimate control at a level previously unseen. Users aren't at the mercy of proprietary vendors to make systems interoperable, instead they're at the steering wheel themselves.
Open Systems impossed limitation on users instead of empowering them. Vendors back then simply realized that it was a clever sales strategy to promise interoperability. In practice it was unattractive and painful for users to try to achieve the promises made.
Open Source in contrast does not make any of these empty and unkept promises. Instead, it delivers control and freedom to users. It has revolutionized software far beyond the horribly failed Open System movement. It creates ecosystems that extend beyond vendor boundries and leads to hyper-innovation. Beyond that, one might claim that it was the catalyst that enabled cloud to come into existance. As a result we should never equate the two.
