We are planning this. In our case, over a period of time, three different data centers evolved to cater to the needs of 3 different groups. Then, as we grew, the lines of separation started blurring. We are now at the point that there is enough justification to consolidate rather than stay separate.
The two big technologies that helped us get to this point are virtualization and SANs.
Consolidation will help reduce some of the complexity in the environment and help us lower power and cooling costs.