In the latest Webmaster Help video, Google's Matt Cutts answers his own question:
Can you explain a little bit about the proposed "autocompletetype" attribute? Is this something I should add on my web forms?
"The basic idea is lots of websites have forms," says Cutts. "You type in your name - your first name, your last name, your address, your street address, your postal code - all this sort of stuff. And it's a real pain to fill out those forms. If you're a business owner or a publisher, the easier you can make it to fill out those forms, the more likely it is that people will do purchases or sign up for your newsletter, or whatever it is that you're interested in. So it's highly recommended that you make it easy for people to fill out those forms."
"You take your existing web form, and Google Chrome (hopefully other browsers will pick this up as well) has proposed a standard called 'autocompletetype,' so you can say, 'autocomplete type is street address'," he adds. "It doesn't change your form elements. Your variables are still the same, so you're only adding. It's not as if any of your forms are going to break. But by annotating your forms with the correct type of thing that you expect people to fill in with the browser's autocomplete, Chrome will know exactly how to fill out your forms."
He says it's a "tiny" amount of work, and as a result, users will "whisk" right through your form. There's no down side, he says.
Also, for some reason, there's a clip from a Who song at the end of the video.