Tomek Onyszko:Have you ever wondered how many objects You can create in Your DIT (actually this was showed some time ago by ~Eric) ? Or how many DC is to many in a domain?
For some time now on one of MS internal distribution groups group of folks were discussing document which describes such limits related to AD (BTW - possibility to be a part of such discussion is one of greatest things related to be an insider). This document is now published. It is short but provides all information, check it out - Active Directory Maximum Limits.