I have a new 2011 caravan and yesterday I stopped on a hill and was slowly taking my foot off the break (with the van in Drive) and noticed the van does not stop rolling backwards. Is that by design?

 

every automatic does that. there is slip from your torque converter and clutch discs inside the automatic when the engine is at low rpm/idling. if there wasn't any slip, your engine would be forced to stop turning at every stop. if you are on level ground, the van should creep forward. the weight of the vehicle on the hill is enough to overcome the little bit of friction at the low rpm and you roll back.

 

Welcome to DF
Mirageman is correct. the weight of the van will roll back on a hill, because of low RPM.