So, what I've gathered from testing:

• If you have this "Anyone with a link" setting, Google with include an "authKey" value along with the links it generates to the items (when you click on "Link to this album."). This key is generated once you set it to "Anyone with a link" security.
• In addition, there is a "view all unlisted albums" link, which you can give out, and has its own special authKey, and will allow somebody who clicks on that link to view all unlisted albums.

So, if you want, you can have authKeys that allow certain people (who you give certain links to, or post the link to certain forums) to view the albums or the photos within, but nobody else. Unless that "authKey" is part of the link, Picasa won't show the album.

It's not perfect security (it's only as good as the people you give the link to, or in this case, the forum you post to), but it at least gives you some control over who gets to see pictures. They aren't publicly accessible, nor do they come up in the search.