Found on MSDN: Visual Studio 11 to Visual Studio 2010 Project Compatibility

This post is mostly so that I can find this information quickly in the future.yinyang

One of the big issues that all development teams have face when working on the Microsoft Development tool s stack is that once you open a Solution or Project in a new version of Visual Studio you are no longer able to open it in the older versions.  This was especially frustrating between VS 2008 and VS 2010 because the only change that was made to the files was the Version attribute changed from “8.0” to “9.0”.  No other modifications were made to the file’s contents.  If you “upgraded” by mistake, you only had to change the version attribute back to “8.0” and you could open the file in VS 2008 again.

In Visual Studio 11 (2012?) Beta we are now able to open certain project types while retaining the ability to open those projects in VS 2010 SP1 IDEs.  This allows the team member the ability to move themselves over to VS 11 at their own pace.  Prior to VS 11, the entire team would have to move over at the same time, since once you upgraded the project and solution you could no longer open it in the old Visual Studio.

The Visual Studio team has posted a Visual Studio 11 Compatibility article that describes the current state of this feature.