We will be adopting a newborn, so attachment disorders aren't something I'm terribly worried about. But, because I'm interested in adoption in all it's forms, I've done alot of reading. I know a little, but not enough to speak very authoritatively.
It is a risk with adopting an older child, but that doesn't necessarily mean that an older child will have an attachment disorder. Even abused children can form strong attachments to the parents who mistreated them. A child who has formed attachments as an infant can form attachments to others, including adoptive parents. Children who never attached to anyone as babies, because of neglect or abuse or instituational care (like an orphanage), will have severe problems attaching to adoptive parents and may never be able to form significant attachments.
I'm going to stop now because I just don't know enough about this. But here are some books to check out. You can order any of them from www.tapestrybooks.com
Attaching in Adoption: Practical Tools for Today's Parents, by Deborah Gray
Adopting the Older Child, by Claudia Jewett
Adopting the Hurt Child, by Gregory Keck and Regina Kupecky
Attachment, Trauma, and Healing, by Terry Levy and Michael Orlans
Parenting the Hurt Child, by Gregory Keck and Regina Kupecky
Building the Bonds of Attachment, by Daniel A. Hughes