The idea is based off a post that Jeremy Miller had about a month ago, having a common, shared interface across several IoC implementation. That would allow library authors to make use of the benefits of a container without taking a dependency on a particular implementation.
The alternative for that is to each library to create its own abstraction layer. A good example of that is NServiceBus' IBuilder interface, or ASP.NET MVC's IControllerFactory. That is just annoying, especially if you are integrating more than a single such framework.
This project was created with the aid of most of the IoC containers authors on the .NET framework, and we have adapters for Windsor, Unity, Spring.NET already, with adapters for the rest of the containers coming soon.
What it is not?
It is not meant to be a complete container abstraction. The reason that the interface is so small (and you wouldn't believe the amount of time that it took to settle on exactly what is going to be there) is that this is not supposed to be the container that you are using. This is explicitly designed to be a read only interface that allows a library to use the container, not some uber container interface (which doesn't really make sense considering the differences between the containers).
Why Service Locator vs. Container? Again, the design is focused on enabling integration scenarios, more than anything else.
I still recommend to avoid explicit service locator usage whenever possible, and to rely on the container and dependency injection. This is not always possible, which is what this library is supposed to solve.
Who should use this?
If you are releasing a piece of code that is taking a dependency on a container, you should consider that as an option. It is generally considered to be a better idea than to force your choice of containers on the clients. The OAuth library for .NET is going to move to this, for exactly this reason.
I am from Microsoft, can I use this?
The code is released under the MS-PL, and all of the work on the project was done by Microsoft employees. That was quite intentional, since this is allow Microsoft projects to make use of that (this is Microsoft IP). If you are a Microsoft employee, you can make use of that. In fact, according to Chris, the next version of the Enterprise Library is going to use this as the container abstraction, so you'll be able to use the Enterprise Library with Windsor and StructureMap and Spring.NET and Ninject and AutoFac and all the rest of the things that I forgot.
I can use only Microsoft products, can I use this?
See previous question.
Where can I find the code
The codeplex site is here: http://www.codeplex.com/CommonServiceLocator