Do you know where does this "System.net.dll" come from? What version is it? The one in the GAC from .NET 4.0 doesn't contain the type "System.Net.HttpWebRequest"?
The reason Orchard copies System.net.dll into App_Data is most likely that
1. you module references System.net in the .csproj file
2. *and* there is a version of System.net.dll in the bin directory of the module, i.e. "~/Modules/foo/bin/System.net.dll"
Does your module really need to reference System.net.dll?