Following the comments to this announcement from 2008 (and as I recall it), the bonus disk space is no longer offered:
http://blog.linode.com/2008/07/31/refer ... discounts/As I understand it, if your account had been grandfathered into keeping pre-existing bonus disk space on your Linode, but that bonus will only apply to that specific Linode server, and not any other Linode servers that happen to be in the same account. The bonus disk space would also be removed once you change to a different payment plan.
Two of my Linode servers were created before this change, and since I've kept them on the annual payment, I'll keep that extra disk space. When I set up a third Linode server, I didn't get that extra disk space, but I did get a 10% discount for paying annually on that server.