For a copy paste solution check this custom Walker out: http://goodandorgreat.wordpress.com/2012/01/12/update-2-using-twitter-bootstrap-dropdown-menus-with-wordpress/
It’s missing one or two things like data-toggle="dropdown" and <b class="caret"></b>.
It should be quite easy to figure that out, but here’s my modified version: https://gist.github.com/1817371
Hope that helps.