The note that Joshua leaves for his father reads: "Dad,/Eaten lunch already/and gone to/The Brooklyn Museum of Art/having fun with Nunu and Lily." If you arrange the first letter of each line in order, it reveals an acrostic for "Death."
When Brad tries to seduce Abby in their bedroom, the peak meter of the baby monitor indicates that Lily is still screaming on top of her lungs, when she is actually sleeping peacefully.
The idea came from George Ratliff. The interview of with him can be read by going to http://www.indiewire.com/people/2007/01/park_city_07_in_12.html. The third question asked will help answer this question.