While I am just beginning to research this topic more in depth, and am by no means an expert, I wanted to share some resources I'm exploring. I'm hoping it helps to organize my thoughts, so much to think about.
MommyLife (Good Montessori based resources, and tips)
Riverbend Down Syndrome Association (A great all around resource, but especially loved the section on HS)
Communicating Partners (Not necessarily a HS site, but great info on being your childs' first, and most influential teacher)
Many of the popular HS curriculum sources have adapted materials for children with special needs. Seton (which I've heard great things about! Has resources for children who need special services)
The Topics in Down Syndrome Books (Love these!)