Another issue with the three tethers in the back of the Ody--the centre tether has to cross through the cargo space. If you need a stroller long ways in the cargo space, this could be an issue... or if you have an upright double. There's no tether anchor on the 8th seat (centre, centre) of the Ody but there is one on the seatback of the centre-centre seat on the Sienna.
Having gone from my 2005 Sienna to the 2008 Odyssey, the things I miss most are what Darren has already mentioned. The Odyssey isn't as wide, the 8th seat is only large enough for a child but not a carseat (and the seatbelt cuts across the neck), and it's a bit louder. However, the major advantage to the '08 Ody is that the Touring comes with the 8th seat for the odd time I might need it, whereas the Sienna Limited doesn't have the 8 passenger option.
The one thing that drives me mental with the Ody is that the doors lock when you drive, but then don't unlock (except driver's side) when you put it into park. I have yet to figure out how to get it to unlock when going into park... It's almost enough of a deal breaker to talk me out of buying another.
If I were buying a new van this year, I'd have a really tough decision to make. I'd love to take the seating/interior/body from the Sienna, and the features from the Odyssey. Maybe Toyota will get smart and come up with a fully loaded 8 passenger option? As far as I'm aware, it still isn't available for '09 but I'm not positive since I'm not in the market for a new car again until next year. {Oh wait, it's DECEMBER already! heehee..} I do think the XLE/AWD with an 8 passenger option would win me over.
As Darren said--the rear impact crash rating isn't as much of an issue as you might think. Even the side impact crash ratings aren't a real huge issue {Unless you're a Neon driver..} Most collisions are frontal collisions, and in driving a vehicle that's larger and higher up from the point of most impacts, with your babies all properly restrained, you're already really well protected. Add front and side impact airbags, seatbelts/child restraints, ABS, and all the other safety features on newer vehicles into the mix, and you're already really very safe. I felt really guilty not getting side impact airbags in my Sienna back in '04 for YEARS..but in 2004 we were still really quite unsure of how the side impact protection was interacting with child restraints. Once we had more information in 2005, I wanted to upgrade my car to be "safer".. but once logic settled in, I realized we were already doing everything in our power to be as safe as possible. I waited to replace the Sienna for the full 3 years I keep my vehicles for and still slept well at night.
-N.