Sad... A while ago I read that it would not be compatible... (months ago)... When R2 came out and E2k7 SP came out, thought it would work like a breeze together. Can't imagine we deployed yesterday for example 15 2008 R2 servers but will have to deploy 3 2008 server to the customers *because* a current OS doesn't support a current messaging system period.
Release date of R2 should have been made simultaneously in my opinion if they wanted to have two current "apps" not compatible between themselves.
We are a major player in IT and cannot tell our customer to wait for E2k10 to be released, we have to install or "push" servers, that's our living and customers wants up-to-date softwares (except when Vista came out)