VS 2012 Update 1
Windows 7 x64.
I did a "Manage nuget packages for solution" and added 2 packages to one project out of 8 in the solution.
I did a Enable Package Restore
I set the check box in options to opt-in to the download for the restore
I even added
<add key="enabled" value="true" />
to the nuget.config file as it was not there (same results with and without)
If I do a REBUILD, the error appears. If I do build after that there is no error. If I wipe out the packages folder, I have to do a rebuild, followed by 2 builds before all packages arrive and the build works correctly.
There is a _DownloadNuGet target in the NuGet.targets file in the .netget folder in the solution, so I do not know why it is unhappy