Reraise confusion

So here was the situation:
After the flop, 5 players are in the hand.
First to act, PlayerA bets $40.
PlayerB calls $40.
PlayerC goes all-in for $60.
PlayerD wants to reraise to $100. This caused some confusion at the table. I said that since PlayerC's all-in wasn't a raise of more-than-half of the previous bet/raise, then it kills the reraising action.

So, it seems that John M was right... ONLY in the case where there are people who have not yet acted is this appropriate... in this case, PlayerD can raise PlayerC's uncompleted raise because he had not acted. However if PlayerD had folded, PlayerA cannot raise PlayerC's all-in because PlayerA had already acted. PlayerA CAN re-raise PlayerD's raise, because PlayerD reopened the betting.

Here are a few links I found:
http://www.pokercoach.us/RobsPkrRulesHome.doc
http://forumserver.twoplustwo.com/showflat.php?Cat=0&Number=8957479&Main=8932714