According with MS Exchange Team Blog, Exchange 2007 will be supported on Windows Server 2008 R2.
“We always talk about listening to customers and sometimes this is written off by many as 'marketing speak'. In fact, we do take feedback seriously and no input is more important to our engineering processes than your voice.”
“Earlier this year we made a decision in one direction, and due to the feedback we have received on this blog and elsewhere, we have reconsidered. In the coming calendar year we will issue an update for Exchange 2007 enabling full support of Windows Server 2008 R2. We heard from many customers that this was important for streamlining their operations and reducing administrative challenges, so we have changed course and will add R2 support. We are still working through the specifics and will let you know once we have more to share on the timing of this update.“