This is a strange one. I'll try to explain in detail so you can recreate.

Lets say you have a fresh dolphin install and this line in inc\header.inc.php reads as follows.

$site['url'] = "http://efriendsrock.com/";

no www in the url.

The you log into the site checking the box to remember you.

So now the site will automatically log you in because a cookie has been set.

Now lets say later you change that config line to add a www to the url like so.

$site['url'] see more = "http://www.efriendsrock.com/";

Once this is done you cannot logout. At least in IE7 you can't. For some reason because the url has changed it seem the cookie cannot be deleted when you log out.

Thus people are forced to manually remove their cookies.

I would call this a bug.