We've released a new version of the Nexus Mod Manager today that provides official support for an offline mode as well as a few bug fixes. While the old NMM worked while offline it was never officially supported. Version 0.31.2, the latest version, provides support for an offline mode as a button at the login screen. If you don't want to login, or don't have a connection, you can use the "Offline" button to use NMM in offline mode. You won't be able to download files or view latest version information of your already installed mods, but most other features within NMM will continue to work.