The initial implementation appears to be targeted to e-commerce sites. Your username and password are not included in the fields that this can populate.
Here are the supported fields:
* Contact: name, tel, email
* Shipping: street-address, address-line1, address-line2, locality, region, country, postal-code
* Billing: street-address, address-line1, address-line2, locality, region, country, postal-code
* Payment: cc-name, cc-number, cc-exp, cc-csc, cc-type
1Password allows one to autofill credit card, address and other fields as well, you know. I use that almost as frequently as the passwords.
I use lastpass for the purpose. But I can imagine baking it into the browser will lead to higher conversion rates especially on mobile.
I don't use autocomplete, I use 1password, which guarantees my information is as up to date as I want it to be for all sites.
Perhaps browser extensions like 1Password can also leverage this to keep your autocomplete data up to date with your main contact/credit details?