Will Visual Studio 2012 interfere/break .NET 4 and/or Visual Studio 2010 if installed side-by-side on the same instance of Windows?
As Reigo said, yes. Here's the link to the official Microsoft page with the information Reigo provided, and more details: http://msdn.microsoft.com/en-us/library/ms246609%28v=VS.110%29.aspx