It seems like a feature introduced in Cordova 2.6.0, at least I just noticed in this version.
At this point I could not find any documentation and I don’t have many details on it, but right now I solved the 404 issue adding a dummy cordova_plugins.json file to the root of my project.
As it expects a valid json file I added the following content to the file: “just a dummy file required by Cordova 2.6.0”