With the default setting of 'Accept cookies: only from sites you navigate to' safari only accept cookies from other subdomains when the tld consist of at least 3 characters.
The main site runs on www.example.com and that site serves ads from ads.example.com. ads.example.com should be able to sent cookies to the client, which does work when the top level domain is .com however with .tld's like .nl .be or any other random two char tld is used then it doesn't work.
<rdar://5923246> - thank you for the extra effort of filing it!
Closing as invalid per our process, as a non-WebKit bug.