I am in the process of working through options for Exchange 2010 with a Parent/Child domain structure. It is a greenfields Exchange environment so I dont need to worry about legacy servers. I would like to host mailboxes on a DAG that is in the Parent Domain and allow child domain user access with a single login. I am pretty sure that this should not be an issue but I cannot seem to find any definitive answers. Any help would be appreciated Thanks

If you're referring to actually AD domains and not AD Forests then it should be fine as long as you have completed all your Exchange pre-req's, such as schema, domain prep etc...Sukh

Yes, Single AD Forest, with multiple AD Domains in a parent/child arangement. Cheers