In theory, putting one child in the center position of each row would be "safest", but practically, it's a pain in the bum, lol.
I have a 2003 Ody, and keep my DD in her Marathon behind the driver, and my older ds in the other captain's chair on the passenger side (he's either in a booster or a Husky...my van/seat arrangement looks the same as Darren's in the link below). Sometimes I put the passenger side captain's chair in the center position, but sometimes my kids need a little more space between them.
The tether anchors for the middle row are under and on the captain's chairs...they look like seat supports, and don't believe the salesman if he tells you the big metal bars on the floor are the tether anchors, they are the SEAT anchors, and tether hooks don't fit on them!
Anyway, here's a page all about the Ody and carseat installation specs, that you should find really helpful http://www.car-safety.org/odyssey.html
I'm sure the information for yours will be all the same, since there has not been any design change in the car for a couple years.
Anyway, I can't tell you how much we love ours...it's really a great van :)
Julie CPS Tech and mom to 2 in seats