I thought this would be a bad idea at first, because it'd break all those "remember me" check boxes, but a lot of browsers can save usernames and passwords and do a good job at prompting the user about it, so I think it might be entirely possible. The biggest hurdle is that there are a lot more cookies out there than username and password forms.

Don't forget flash cookies.

And there's precisely one vendor in charge of the defaults and capabilities of those.