WordPress User Authentication

Have any of you used the WordPress user authentication either outside of
Wordpress and/or on other WordPress blogs on the same server?

What I’m wanting to do is create a single-signon for both of these
scenarios…one table of users, not multiple tables for each
area. Separate tables for the rest of the data is fine.

This is somewhat similar to “multiple blogs”, but with the addition of
being able to protect other non-blog areas.