2008-R2 support for Exchange 2007 is much appreciated but I don't think that the lack of R2 support would have prevented anyone from deploying Exchange 2007 servers.
What is more serious is no support for 2008-R2 DC's and 2008-R2 domain functional levels when an Exchange 2003 server is present. In our case the public folder management tools in 2007 lack some of the basic functionality of the 2003 GUI. So we are stuck with 2003 public folder servers until we can find a suitable replacement. You really cannot expect help desk and admin users to use the 2007 command line to set a wide variety of MAPI folder permissions on 10's of thousands of public folders for thousands of users on a day to day basis.
No Exchange 2003 support with the aforementioned R2 scenarios means that R2 domain controllers cannot be deployed and you cannot get the benefits that would be realized by raising the domain functional level to 2008 or 2008-R2. Thats a big deal.
Any thoughts to reconsidering Exchange 2003 support when 2008-R2 DC's are present or the domain functional level is raised to 2008-R2 ?