100% cloud, 100% physical or hybrid?

Overview

In the vast majority of enterprise data centres, cloud-like architectures are quickly taking root. Companies are virtualizing their resources and partitioning some of their applications within their four walls. However, risk-averse enterprise IT professionals are understandably cautious about simply moving their entire IT portfolio of resources and services into a 100% architecture. Not everything will move to the cloud - at least not in the foreseeable future. That's why so many IT professionals advocate a so-called "hybrid" approach. But what do we mean by "hybrid" and how do we optimally balance the various components of hybrid cloud architecture?