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.

BTW - if this just happens that you are a customer who is near of such limit or had actually crossed it (if this is related to number of objects - call PSS :) ) I would be happy to hear about it either through comments or through e-mail. So far I worked with one particular customer who has actually crossed one of such limits - this was about number of DCs in single domain. But this isn't hard limit - is it more like recommendation and if You know how to handle this You can live with it.

So ... another interesting reading.