A. Exchange 2010 is supported on the 64-bit editions of both Server 2008 and Server 2008 R2, but you can't mix the OSs within a single DAG. This limitation is because DAGs use Windows failover clustering behind the scenes, and failover clustering doesn't support a mix of Server 2008 and Server 2008 R2 nodes in the same cluster (because of differences in cluster implementation).
If you want a mix of Server 2008 and Server 2008 R2, you'll need to have multiple DAGs, each containing one version of Windows.
Related Reading:
Check out hundreds more useful Q&As like this in John Savill's FAQ for Windows. Also, watch instructional videos made by John at ITTV.net.