How can I automatically attach posts to a map?

You could use this plugin, although it may only be compatible to 3.2.1. And this plugin may help you out, although it seems a little complicated to implement a map for all the posts.

You could also use a custom field for lat/long on each post, and then create markers using wpdb calls, grabbing the post_meta data, and using the Google Maps API to place the markers, but it sounds as though you’d prefer an out-of-the-box solution. If you do want to attempt this, you might use the above plugin’s code as a development roadmap.