So I have created a bug report here for this same issue, but haven't heard anything. When using an optional node reference field, I am unable to save with "None" selected. It throws an "incorrect integer" sql error. This is obviously due to it trying to insert the record without a correct int nid selected. Any ideas how to fix this, it is fairly urgent for an upgrade I am working on. I have searched the node reference code base, and the _none doesn't seem to be set there, and I am trying to avoid patching the core fields.