It is important to note that the RTM version of Exchange 2007 is not compatibile with Server 2008 RTM. You must have Exchange 2007 SP1. Exchange 2007 SP1 is not just a service pack, it is a full install of Exchange with the Service Pack slipstreamed into it and is not available as a standalone Service Pack.
Exchange SP1 brings in a number of major architectural changes, it is not possible to upgrade 2007 RTM to SP1 and then upgrade the OS to Server 2008. This will break Exchange completely. It is also not possible with a cluster, you cannot do a rolling upgrade. If you have a cluster set up you must build a new cluster with Server 2008 and then Migrate your data from the old cluster to the new cluster.
The only path is to perform a Migration.
Obviously you will need to deploy new hardware to install server 2008 and then Exchange 2007 SP1. Once you have your new server in place you can install Exchange SP1 and then migrate your data from your other Exchange.