As this question still is rised on the newsgroups I've diecided to blog a little update to my previous post regarding extending the schema for Windows 2003 R2 DC. Since the date I've posted this message KB 917385 article was published which describes this behaviour and gives a solution as well.
As some of you may remeber I wrote also second post in this topic which dealed with extending the schema from x64 version of Windows 2003 R2 when You don't have other 64-bit DC in Your network.
What is strange we can read in this KB:
Installation disk 2 contains installation files for the x86-based version and the x64-based version of Windows Server 2003 R2.
Maybe this is true for newer media which comes for WIndows 2003 R2, it isn't on my Select CDs. Maybe somebody will be able to check it and give me feedback regarding this statement.
Unfortunatelly KB article which describes this issue with 64-bit and delivers both versions of ADPREP as separated downloads is still under development, but I hope it will be published soon.