Why wouldn't you just run everything off of exchange 2007? The profiles/permissions should be moved with besadmin from 2000 to 2007 and the only thing you're going to have to change is the mapi address of the exchange server.
Exchange 2007 is solid.
Mitchel Lewis - (Sys, BES, Exchange, SAN, Network) Admin
Current: 8900 on .168
Exchange 2007 and Blackberry Enterprise Server