Windows 2008 Small Business Server installation

Also, the Win 2003 servers are set as domain controlers, one master and one backup.

To do this you need to create the domain with the SBS server, you can then add the other servers to the domain and add them as additional DCs - its extremly complex (impossible) to add an SBS server to an existing domain as the SBS server must hold all of the FSMO roles.

Additional DCs (win2003 or 2008) can be added to an SBS server 2003 or 2008 domain and there will be no problem if you do it this way around

BTW there has been no such thing as a Master and Backup DC since Windows NT4 !
They are all DCs

I take some of that back - you can add an SBS2008 server to an existing 2003 domain, however, as I said you need to ensure that the SBS server has the FSMO roles - its a bit complicated - but not impossible - see http://technet.microsoft.com/en-us/library/cc708131(WS.10).aspx

See this similar question https://www.experts-exchange.com/questions/23897813/Joining-Windows-Server-2008-SBS-to-existing-2003-domain.html

All I want is the MS Exchange portion of SBS.  Would it be better to just purchase 2008 standard and a copy of MSExchange?

The only reason I see that the box needs to join the domain is for Active Directory integration.

Thoughts?

And yes, I know the PDC and BDC are gone, just hard for me to think of it any other way :O)