It is a compulsory requirement that you need a license to work/operate a software application. That means it is required for each physical or virtual processor accessed by an operating system environment running such a software.

I must admit  Licensing policy or information from Microsoft is bit confusing, many times I was stumbled upon what to refer or review in case of any deployment of SQL Servers within a physical or virtual environment. So I tend to visit HowToBuy all the times to make sure I do not violate or overpay any financials towards licensing.

Open Source software has taken more importance to the IT users, one way it is good for variety of technologies to use. Until now the world was waiting to see what would be the stand from a Software gaint such as Microsoft. No doutb that Microsoft has to protect their own software in addition to contribution towards open source, an interesting blog post I have seen that is interesting to know that a number of agreements with various software companies to protect them and their users and list of strategies that are credited some other concrete ways method here.

Here is the interesting information in their (MS) own words about Open Source at Microsoft: Licenses: Microsoft Public License (Ms-PL) policies.