If you decide to use Windows Clustering, to deploy Exchange 2007 (RTM or Sp1) in a CCR or a SCC topology, you need to make sure that you have a Enterprise Edition license for every node which will be part of your cluster, even for the passive nodes.
For more information, have a look here: http://technet.microsoft.com/en-us/library/bb232170(EXCHG.80).aspx