I was playing around with Nuget trying to figure some things out and I ended up trying
<file src="bin\$configuration$\*.dll" />
Pack command usage: Nuget.exe pack C:\Projects\zzz\Core\Core.csproj -Configuration Release -Verbose -OutputDirectory .\bin\
This some how results in Nuget packing files from \bin\Debug\ and none from \bin\Release. I don't know what should happen here, but I know it shouldn't have been that.