I realize this post is very old, has been revived twice already, and has been addressed in another interpretation thread as not allowing two standard actions. However, I just ran into this in my game today, interpreted it as granting two standard actions, realized it was broken, and found this thread researching it. It seems confusing, but I think I thought up an interpretation that makes sense, so I wanted to post it for anyone else looking this up later.
It works as intended if you consider the Flyby Attack as the standard action. That is, in any situation where a monster would get a standard action (such as a surprise round, is a zombie, is slowed, or gets a normal round), the standard action can be a Flyby Attack. By using up the standard action, Flyby Attack has to grant another attack within itself to be at all useful. The intent of giving the dragon strafing ability and the fact that the breath weapon isn't an attack means that they had to state it as a standard action.
So, it might read "When flying, the creature can take a move action and another standard action (in addition to the one used to initiate the Flyby Attack) at any point during the move." The line "The creature cannot take a second move action during a round when it makes a flyby attack." prevents monsters from getting two moves and an attack.
I guess it is also implied that the standard action inside the Flyby Attack cannot be another Flyby Attack. Otherwise, that could be chained to get infinite movement.
But this point of view interprets the Flyby Attack as a standard action just like any other Special Attack in the 3.5 PHB (pages 154-160).
Interesting point that the Flyby Attack doesn't avoid the Attack of Opportunity like Spring Attack and Ride-By Attack. Also, Ride-By attack allows the double move with the attack.