Changes the cookie default path to / (i.e. the whole domain.com not just domain.com/blog) with an option to go across subdomains
You don't need to, by default the path is removed from the cookie just by enabling it
This is new as of version 1.5, root cookie can now change the wordpress cookie to be accesiable across subdomains. e.g. cookie set as http://www.domain.com becomains domain.com
In the wordpress administrator dashboard, select root cookie and tick the box!
In the wordpress administrator dashboard, select root cookie and unselect "Allow Cookies to go across All Subdomains" and in the "Domain Name" box type in you domain such as domain.com or domain.co.uk
Try maunally setting the cookie, see above.