Ignore it and if a hotmail customer complains to you, tell them their hotmail SPAM filter is busted and you can not do anything against it. The hotmail users should start yelling at Microsoft and not at anybody else.
I wouldn't see it so bad. First, they'll start in november which leaves some time to the rest of the world to adapt to the situation. Second, I think it's a good point to promote SPF ( I know we already hat the discussion about whether SPF is good or bad ). Third, it seems as if the message will only be tagged as spam and not deleted or rejected. It's like giving spamassassin a high score for SPF.
With AOL and now Microsoft (counting only the really big ones) adopting SPF I see a real chance for this technology to be successfully used.
Best regards,
Jean-Pierre