A little late to the party, but NodeJS (as of ~Nov 14 I guess) supports corporate NPM repositories - you can find out more on their official site.
From a cursory glance it would appear that npmE allows fall-through mirroring of the NPM repository - that is, it will look up packages in the real NPM repository if it can't find one on your internal one. Seems very useful!
npm Enterprise is an on-premises solution for securely sharing and distributing JavaScript modules within your organization, from the team that maintains npm and the public npm registry. It's designed for teams that need:
easy internal sharing of private modules better control of development and deployment workflow stricter security around deploying open-source modules compliance with legal requirements to host code on-premises npmE is private npm
npmE is an npm registry that works with the same standard npm client you already use, but provides the features needed by larger organizations who are now enthusiastically adopting node. It's built by npm, Inc., the sponsor of the npm open source project and the host of the public npm registry.
Unfortunately, it's not free. You can get a trial, but it is commerical software. This is the not so great bit for solo developers, but if you're a solo developer, you have GitHub :-)