Added to official ideas list at http://drupal.org/node/234712
(Draft SoC 2008 project proposal)
The framework must be pluggable for different OS and browsers (i.e. IE6/7, Firefox, Safari, Opera, Konqueror).
http://wtr.rubyforge.org/ (WATIR browser automation library, written in Ruby)
Simpletest automation was the product of a 2006 SoC project: http://drupal.org/node/58881
http://browsershots.org does a similar thing for css/markup on multiple platforms