Been trying to add a 2008R2 server to a 2003R2 SP2 Domain, it works OK. Will be putting Exchange 2010 on it and adding it to a customers production environment, will update with all my tips, tricks and learnings here in the not too distant future 🙂
However adding a 2008R2 DC is another bag full of kippers, follow this in principle http://technet.microsoft.com/en-us/library/cc733027(WS.10).aspx and you should be OK! Just gotta do the usual domain/forest prep so the 2003 Domain can get along nicely with your shiny new 2008R2 box.