In late May, Microsoft announced that the release candidate version of Windows Server 2012 was available to download for free. Usually, a release candidate version means that a software product is almost ready for prime time but the company still has a few more bugs to deal with before it makes it official.
However, it looks like Microsoft is confident enough in the current version of Windows Server 2012 that it is already being used by one of its largest public facing divisions. In a post on the official Windows Server blog, Mukul Sabharwal, one of the software development engineers on the Bing team, states:
The promise of the new Windows Server 2012 features intrigued the Bing.com team as we considered migrating to the latest operating system. What began as exploratory evaluations of the impact of a migration quickly led to a full-scale deployment, which benefited greatly from the built-in .NET 4.5 functionality, multicore JIT functionality, and potentially, the much-improved Hyper-V 3 functionality.
The end result of the Bing team's evaluations of Windows Server 2012 is that the release candidate version is now being used by the Bing team. As Sabharwal proudly announces, "All Bing.com search results worldwide are being served by Windows Server 2012!" So keep that in mind when you make a search request on Bing.com; it's currently being generated with help from server software that technically isn't finished yet.
Source: Windows Server blog