Active Directory - Organizational Units or Discrete Domains?

flosofl asks: "I work for a large (1,000+ emp.) company and will be in charge of its Active Directory implementation. Our company is in turn owned by a much larger corporation (15,000+ emp.), but we are for the most part autonomous in terms of managing our internal IT dept. Since the larger corporation has ADS in place, they want us to roll in as an OU in their domain (xxx.com). I want to be a child domain (yyy.xxx.com). The SAP portal relies on LDAP and we are told it would not work correctly with a multi-domain model. I on the other hand want total control over MY domain (yes, I know as a parent domain they could do what they want - the illusion is enough). My question is, has anyone been in this type of situation before? How did you resolve it, and did it work? I am worried I am reacting more from a 'you can't play with my toys' than a legitimate tech/business reason. I want to use the method that will work best (which may not be the one I want). Any comments would be appreciated."