There are occasions when you need to extend the Active Directory schema for applications. Besides the best practice of test, test and test again, and the alternative of using ADAM (Active Directory in Application Mode) to avoid changes to your corporate schema, sometimes needs must. In these occasions, you should apply for unique OIDs (Object IDs) which can be done through MSDN at Microsoft where you can have OIDs registered guaranteed to be unique in the microsoft.com namespace. You can of course register OIDs in your own namespace too.
Follow these links to find out more: http://msdn.microsoft.com/library/default.asp?url=/library/en-us/ad/ad/obtaining_an_object_identifier.asp and http://msdn.microsoft.com/library/default.asp?url=/library/en-us/ad/ad/extending_the_schema.asp
I have a good article on this at http://www.depicus.com/articles/extending-the-active-directory-schema-for-enterprise-printer-support.aspx
Nice article, Brian.