My online group and I were doing an APG playtest when the Cavalier's player noticed some discrepancy in our understanding of the rules.
Since your mount is the one making the charge, it must end up in a space it can make an attack from, which usually means a reach weapon can't attack since you must wind up adjacent to the target. One could also consider the mounted pair to be their own entity, with the mount stopping as soon as the rider's weapon can attack with its reach (or perhaps choosing one or the other). That keeps both of them from attacking, though, which may or may not be the intent.
I'm mostly curious as to whether there's an 'official' stance on this (be it actually from a designer or just widely accepted), as it seems to be the only part of the mounted rules I can't fiddle through to the satisfaction of myself and my players.