My thought on this is it is better to move all to Exchange 2007 rather than leave 2003 in the mix. Exchange 2007 and 2003 communicate differently regarding meetings, free/busy, public folders and account creations etc. It is better to have all on the same version while you await the green light to move to 2010. From my own experience migrating from Exchange 2003 to 2007 the time period where it was mixed was painful for a lot of users and caused a lot of scheduling and calendar headaches until we were all on 2007. OWA is much different as well.