I have a windows 2003 domain and an Exchange 2003 server. Patched and current. We are moving to Server 2008r2 and eventually Exchange 2010.
I have read about issues with Exchange server '03 in a 2008 domain. I also read it is "supported" in an R2 domain. It seems there is confusion over how well it is supported in R2. One issue I've read about is creating new users and mailboxes.
I was planning to add new 2008 r2 servers, migrate the domain, terminate old DCs. Then in roughly a month, migrate to exchange 2010.