I talked to Jason Buffington, senior technical product manager for Microsoft System Center Data Protection Manager (DPM). Because DPM is so new and has several third-party competitors, I asked him in particular about the product's licensing.
Each DPM 2007 management server will require a server license, and each managed OS environment will require a management license. Jason explained that there are only three SKUs. The server SKU runs on the back end and lists for $722, which includes SQL Server 2005 under the covers. (No license or CAL is necessary for DPM’s instance of SQL Server.) Alternatively, you can acquire server SKU on an OEM appliance with Windows Storage Server. On the front end, the management license is available in a Standard Edition (aka "file agent") for the file workload, priced at $199, and an Enterprise Edition (aka "application agent") for everything else, priced at $539.