Please make sure to check the following:
Ensure npm login (or npm adduser) was executed successfully,
Ensure the .npmrc file contains authentication info for the feed you are publishing to,
Ensure always-auth is set to true for this registry (see http://docs.myget.org/docs/reference/myget-npm-support#Fixing_401_Unauthorized_after_running_npm_login)
Also note that packages.json must not contain a publishConfig property. If it does, make sure it points to the correct registry or remove the entry.
The npm client will always push to the registry specified in this property, no matter which registry is specified on the command line.