"The type or namespace name 'Linq' does not exist in the namespace 'System' (are you missing an assembly reference?)"
I was getting this error when using a NAnt script to build a project I created in Visual Studio 2008. Turns out to be related to which .NET Framework is targeted, in this case.
Aside: If you are not using NAnt, and you're getting this error when you try to build, then make sure your project has a reference to the System.Core assembly. But if you use the defaults in Visual Studio 2008, then you already have this reference. My project would build through Visual Studio, but not through my NAnt script.
Here's the cause. I thought I should use the latest stable release, instead of the latest beta, so I downloaded NAnt 0.85. It supports multiple frameworks, and by default it targets the framework in use. You can point it to another using the -t command-line argument.
But NAnt 0.85 is aware of frameworks only up to 2.0. For LINQ, you need 3.5, and therefore you need NAnt 0.86 (in beta at the time of this writing). I kept 0.85 just in case, so my NAnt folder contains a folder for each version; they co-exist. I just changed the path in my nant.bat file to point it to 0.86-beta. And voilà.
Hope this helps. Yay, automated builds!