While we wait for Google to post official notice of its support for the new nofollow attribute, Yahoo's already chimed in on its blog that it will do so as well. And apparently, the Google announcement may come here, as Yahoo is already linking to it. MSN tells me directly it also will support the tag, and plans to post on its blog as well. As with Google, Yahoo's linking to where that will likely show up. Ask Jeeves tells me it is still considering the tag. More to come in a long story I'm about to post!
Postscript: Support has now been officially announced by everyone. See the Google, Yahoo, MSN Unite On Support For Nofollow Attribute For Links for further details.