Thanks for the opinions, guys.
I was thinking about it some more, and I came to this conclusion:
We can interpret "at the beginning of combat (before any model attacks)" in a few different ways. We'll just have to see what consequences each way entails.
1) At the beginning of combat = the very beginning of combat (just after the initial charge of the first unit to attack, perhaps multiple turns ago). This means that new units that charge cannot be attacked on the turn that they charge, or, for that matter, at all (at least until the combat is no longer a multi-unit combat and the p41 rule ceases to apply). However, this seems wrong - my Wraithlord assaults a unit of Marines and then I assault the Marines with Guardians which they aren't allowed to attack until the Wraithlord is dead? In fact, if the Wraithlord did die but before that, I assaulted with a 2nd unit of Guardians, the Marines would have to stand there, unable to attack, since the only unit that "started the combat" with them was dead! Thus, the interpretation cannot be 1.
2) At the beginning of combat = anytime from the beginning of the turn to just before attackers move. This is the only way that you can get the result of can't attack units that charged me this turn but can attack them later. However, it would also mean that the newly charging unit couldn't dish attacks, either! They wouldn't be engaged with the defending unit yet! Thus, the interpretation cannot be 2.
3) At the beginning of combat = after the "Defenders React" but before any attacks are dished. At this time, the newly charging unit is engaged with the defender, and so can dish attacks, however, the defender is also engaged with the attacker, and so can dish in response. This interpretation is possible.
I don't see any other possible interpretations for "at the beginning of combat." Thus, the only acceptable interpretation is after "Defenders React" but before attacking, and this would mean that all units in the combat (barring an aforementioned ABCD situation or other lack of base to base contact) can attack all units in the combat, including defenders attacking the attackers that just charged.
Thanks again for the help!