I half-heard this on the radio this morning. (And, damn, I left my MP3 player -- which includes an FM tuner -- on the dining room table. No NPR until I drive home.) I then picked it from Sarah Houghton (Librarian in Black) who links to the ars technica blog.
Will it stop spam? Not much based on my glances at the daily "audit" of the email coming into my work mail boxes. It sure seems like the spam volume fluctuates. Right now I am back to a one-to-one ratio of spam to real messages (on the more widely broadcast of my two work emails).
But until Hotmail does this it will not help me! I don't think I have received more than one or two AOL or Yahoo spam messages in any given day for more than several months.
It will be interesting to see where this goes.