Thanks Kimmy for bringing this to my attention!
Auto-login should be more persistent now, also forum is not trying to validate browsers info string which means more compatibility with IE7 and browsers using all sorts of toolbars and other useless crap. Cookie name also changed, so you need to login again. If you want you can delete old cookies for vogonhq.com domain, to keep things clean.
Tech info for webmasters: PHP 5.3 adds more security and restricts passing of cookies via $_REQUEST variable.
If you wish to change default behavior, find request_order = "GP" and replace it with request_order = "GPC" (C = cookies) inside php.ini and then restart your web server.
In case request_order is not set, variables_order will be used, which is usually set to GPCS by default.