It's a security hardening mechanism.
Essentially, it helps protecting against CSRF type of attacks where the attacker reads a JavaScript array (downloaded as Json) from a victim website. They can do that by overriding JavaScript's Array type. d causes the returned Json to not be an array and thus turns Array overriding useless for the attacker.
See this great blog post: http://haacked.com/archive/2008/11/20/anatomy-of-a-subtle-json-vulnerability.aspx
