however, most of the "do not use" info was dated pre year 2000, after that most of the info leads to "inconclusive" results.
Apparently the IIHS and the NHTSA started to really test this things about 5 years ago and they all say that while it works perfectly for adults, it's hard to determine for children, due to the different types of Side Airbags and the position of the children during the impact; some studies say that if the child is sleeping agains the window, a top-down side curtain, might not help enough (reach) due to the kid's size. Most of the places refered to consult the specific car seat manufacturer specifications to this subject.
The interesting stuff I found is that there is a law in the make that will be set in place starting Sept/2009 to require all car manufacturers to be equiped with Rear Side Airbags, and that this can be activated or de-activated by dealerships and/or manufacturers depending on the order.
Particulary the IIHS maintains a VERY updated list of car brands and models equipped with rear side airbags, particulary the Q5 is STANDARD equiped with rear side airbags for the head, and not available for the chest... Do I hear VAG-Comm hack comming???? The link is attached. <EDIT> No need to hack, it's indeed standard side curtain protection, stated on the brochure</EDIT>
Though, not to create a controversy, and based on my quick Google search ("rear side airbags child safety"), it seems that the recommendation is indeed, if you can, get them because the dealer can de-activate them, or just place the kiddo on the center seat, until it grows a bit.<ul><li><a href="http://www.iihs.org/ratings/side_airbags/side_airbags.aspx">IIHS cars with rear side bags</a></li></ul>