Slashdot writes about the fact that both SPF and Sender ID were accepted as experimental standards by IETF.
IEFT is the organization that handles Internet standards. SPF and Sender ID are two technologies to stop spam. In short, they require sender to authenticate. It means that it's easier to check if the message is spam or not.
The thing is that Sender ID uses many patented technologies and cannot be used in Open Source products. SPF, on the other hand, is already implemented in many programs, and free of patents.