There's a good discussion of the issue of driver reaction time and the factors affecting it (with a couple of references) here: http://www.visualexpert.com/Resources/reactiontime.html
The best estimate is 1.5 seconds for side incursions and perhaps a few tenths of a second faster for straight-ahead obstacles.
Based on that article, I would say that the two second rule should be considered a minimum distance at which to follow and the following distance should be increased as factors dictate. For example, if you're tired, your reaction time will be longer. Allow yourself more distance. If it's dark or rainy or the road is slick, your ability to see an obstacle and stop will be degraded. Allow yourself more distance. If you have a talkative companion in the car distracting you, your cognitive load will be higher and your reaction time will be longer. Allow more distance.
While it probably doesn't have much relation to one's ability as a driver, a fun way to test one's raw reaction time is here: http://www.humanbenchmark.com/tests/reactiontime/
