I do know that Moon Design's HeroQuest Pavis book has slipped by about a month (though pace the above remarks, I don't know the actual dates for this either) due to ISP problems. I also know that when the books were originally talked about both companies said they would be working together to ensure a degree of consistancy between the 2nd and 3rd age city, so I will idly speculate that there may be some sort of "gentlemans agreement" to release the books at the same time?
Its a nice thought, but its not the reason. 'Pavis Rises' was put back so that we could get 'Cults' and 'Races' out first and conduct some review of the 'Pavis Rises' content. But, the manuscript's in editing and knowing the pace for such things, I think October's a more realistic date than September.
We did work very closely with Moon, BTW. 'Pavis Rises' was shared with Jeff and his team, and I've had a hand in their Pavis book too, so the synergy's close - but there was no conscious attempt to release both books at the same time.