<p>Visual Studio Solution files contain two GUID's per project entry. I figure one of them is from the AssemblyInfo.cs</p>

<p>Does anyone know for sure where these come from, and what they are used for?</p>

According to MSDN
:
[The
```
Project
```
] statement contains the
  unique project GUID and the project
  type GUID. This information is used by
  the environment to find the project
  file or files belonging to the
  solution, and the VSPackage required
  for each project. The project GUID is
  passed to IVsProjectFactory to load
  the specific VSPackage related to the
  project, then the project is loaded by
  the VSPackage.