All Things D published an article today about why the data center industry ought to adopt a JIT approach to building data centers. This is effectively what Microsoft is doing in its new Gen 4 data centers where it has a global supply chain of manufacturers who build different modules, e.g. power modules, IT modules, cooling modules, etc that can be assembled in different configurations to deliver different classes of service. Now, rather than building a huge custom-designed facility and gradually filling it to capacity, Microsoft can have JIT approach to adding capacity, allowing it to respond quicker to demand signals from its various properties while delivering outstanding PUEs. Moreover, these new Gen 4 data centers are significantly less expensive for Microsoft to build and operate in part because less of the site is being aside for electrical and mechanical equipment and they're being cooled by air-side economizers. In its newest designs these modules rest outside on a concrete pad.
Modularity is not a panacea though. There has to be uniformity and standards for it to work well. Additionally, you need an application that is highly tolerant of hardware failures because the module that houses the IT equipment is now a fault domain. Consider what would happen if there were a fire inside a module. Also, how easy will be to retrofit the container with new gear? How much up front engineering will be required to accommodate a modular design? These are things you'll want to contemplate before moving forward with a modular approach.
There have been many articles written about the pros and cons of modular data centers, including a a video of Kevin Brown, the author of the All Things D article, from October 2012 when he was speaker at the Data Center World Conference.