Although there is a relative Reference HintPath in the .csproj file, when you move a solution to a different machine, the hint path may not be correct for that machine.  An easy way to get around this problem is to just ensure that your needed assembly is in the Public Assemblies folder: Program Files\Microsoft Visual Studio .NET\Common7\IDE\Public Assemblies.  Of course this should really only be done when you don't make any changes to the referenced assembly, such as a 3rd party dll.

A good summary: http://blogs.msdn.com/manishagarwal/archive/2005/09/28/474769.aspx

From: http://msdn2.microsoft.com/en-us/library/8y13ka7c(VS.80).aspx

When the project system finds an assembly reference, it resolves the reference by looking in the following locations, in the following order:

1. The project directory. The project directory files appear in Solution Explorer when Show All Files is not in effect.
2. Directories specified in this dialog box.
3. Directories displaying files in the Add Reference Dialog Box.
4. The project's obj directory. (Any assemblies created as a result of adding a COM reference to your project are added to the project's obj directory.)

From: http://msdn2.microsoft.com/en-us/library/wkze6zky(VS.80).aspx

To display an assembly in the Add Reference dialog box

* Move or copy the assembly to one of the following locations:

o The current project directory (you can find these assemblies using the Browse tab).
o Other project directories within the same solution (you can find these assemblies using the Projects tab).
o The Public Assemblies folder: Program Files\Microsoft Visual Studio .NET\Common7\IDE\Public Assemblies; (you can find these assemblies on the .NET tab).

* Set a reference path to the directory containing the assembly using the Reference Paths Dialog Box (Visual Basic) or the Reference Paths Page, Project Designer (C#, J#).
* Set a registry key that specifies the location of assemblies to display (This works with MSBUILD):

Add one of the following registry keys, where <AssemblyLocation> is the directory of the assemblies that you want to appear in the Add Reference dialog box, for example, C:\MyAssemblies\.

[HKEY_CURRENT_USER\SOFTWARE\Microsoft\.NETFramework\<version>\AssemblyFoldersEx\MyAssemblies]@="<AssemblyLocation>"

[HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\.NETFramework\<version>\AssemblyFoldersEx\MyAssemblies]@="<AssemblyLocation>"

Basically the AssemblyFoldersEx key can have lots of subkeys.  The subkeys can have any name.