Let's use Chiliproject.org as an example:

When I log into Chiliproject.org without the Remember Me checkmark, the site sets 4 cookies on Firefox:

1. _chiliproject_session, expires at end of session
2. __utma, expires 25 september, 2013
3. __utmb, expires 26 september, 2011
4. __utmc, expires at end of session
5. __utmz, expires 27 march, 2012

I don't really known why the __utm<x> ones exist, but I am interested in the _chiliproject_session one anyways. Notice that it expires at the end of the session.

When I log into Chiliproject.org with the Remember Me checkmark, the site sets 5 cookies on Firefox:

1. A blank-name cookie, expires 26 september, 2012
2. _chiliproject_session, expires at end of session
3. __utma, expires 25 september, 2013
4. __utmb, expires 26 september, 2011
5. __utmc, expires at end of session
6. __utmz, expires 27 march, 2012

Now, I imagine that the blank name cookie is the one that is used by the Remember Me functionality. I do not understand why its name is blank (or undefined, I imagine). Also, when I leave my browser, and restart, that blank-name cookie is still there (makes sense) and when I go back to Chiliproject.org, the site still asks me to authenticate. That brings me to believe that chiliproject.org as a site doesn't permit autologin. If this is not the case, then the current site code is broken....

If we really set a blank name for the Remember Me cookie, then I think my Safari 5.1 doesn't set cookies with blank names, and that would explain why I can never autologin even with a 365 days autologin period set in CP prefs.