This is another popular doc. I have seen it at a number of customers blown to A0 size and mounted on a wall!
You can get it here: http://files.flaphead.dns2go.com/exchange_2000_Message_Flow_v1.4.zip
It still applies to Exchange 2003, but NOT Exchange 2007.
You can get the Exchange 2007 Exchange2007_EdgeTransportRoleArchitecture and Exchange2007_HubTransportRoleArchitecture ones here: http://www.microsoft.com/downloads/details.aspx?FamilyID=612f811d-2953-4c08-945e-833c17150083&DisplayLang=en