What i have noticed is a lack of feet movement (or rather, some poor feet movement) to some balls outside off, particularly those full and with decent pace. However whether thats necessarily completely technical, or is mental (I didn't watch enough of Ind vs. SA nor Eng vs. SA to properly give my opinion) I am unsure.
When would you suggest a rest? If you look over his recent scores, he's managed to squeeze some decent to good knocks here and there. Poor England series of course, but he did cross 50 once (which is obviously crap for Kallis), and he tonned up in an important match vs. India. Can't quite recall what he did vs. West Indies, but I do recall some runs being scored too.
The thing is, I've always thought of Kallis, even more-so than Dravid (because of the mental strain Dravid has to go through being an Indian cricketer and captain/former captain), that Kallis is the most likely to continue churning out runs no matter how old he gets, because he has such a solid technique and strong desire to score runs. I still feel (though less confident) that he'll take the record for most test runs by the time he retires, because unlike Ponting (who relies on more natural talent, his eye and poise/time when playing the ball) Kallis is manufactured solely on his technique.