News is that MS plans to use it on Hotmail. Anything not having Sender-ID will be tagged as spam.
Why is this move significant? MS is going to activate their version of a Sender-ID system even while talks are ongoing about establishing industry anti-spam standards. But the IETF standards group has already rejected the MS Sender-ID proposal... which MS is now going to activate anyway.
This is, in fact, a brilliant (and evil) strategic foray by Microsoft. If they can leverage their Hotmail influence (and alliance with AOL) and push Sender-ID into becoming an ad hoc standard (so average users can email Hotmail and AOL users), then they have effectively added their own protocol to the Internet standards.
And they own the patent! The Apache Foundation has analyzed the situation and concluded that open source cannot implement MS Sender-ID since it represents proprietary technology. If Sender-ID becomes a de facto email standard, then Linux will be crippled as a serious commercial platform.
Now, realistically, I think that the MS move will likely be blunted by lack of industry acceptance. But it is still a devious and daring attempt that bears watching.
 Yes, I'm aware that this sort of maneuver is nothing new. But it's often attempted on emerging Internet services rather than established protocols. And, even if it's nothing new, we have to be aware every time stuff like this is attempted.