Overview of operating system and Active Directory requirements for Exchange
Server 2003
http://support.microsoft.com/default.aspx?scid=kb;%5Bln%5D;822179
Also check:
http://www.microsoft.com/technet/itsolutions/mobile/deploy/msfpdepguide.mspx
There's no dependency on Windows Server 2003 DCs/GCs iirc.

Signature
Bharat Suneja
MVP - Exchange
www.zenprise.com
NEW blog location:
www.exchangepedia.com/blog
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>I need to upgrade to Exchange 2003 to support TREO's wireless email
> functions. At this time, I don't have the budget to upgrade my whole AD
> domain to Windows 2003. Will Exchange 2003 fully operate on a 2000 AD
> (and
> support wireless email)? Thank you for the help.