Shared authentication is a popular but tricky problem to solve. Another often-requested feature for multi-site installations is sharing content.
The multi-site design was not initially intended to share content. In fact, it was intended to not share content. Be that as it may, there are a few ways of accomplishing this.
As with shared authentication, one possibility is to attempt to share database tables between different sites on a multi-site. On occasion, users report having attempted this method on their own sites. But this method is also fraught with difficulties. Security is difficult, updates require special care, and there is constant danger of what is called a "race condition", where two different sites vie for control of the same resource. Consequently, this method should be avoided.
There are a few other more reliable methods, though.
The Services module (http://drupal.org/project/services) discussed in the previous section...