From DeveloperWorks, Make SOA real with IBM WebSphere Enterprise Service Bus and IBM WebSphere DataPower SOA Appliances, Part 1: Use WebSphere Enterprise Service Bus for protocol switching of encrypted data

And Make SOA real with IBM WebSphere Enterprise Service Bus and IBM WebSphere DataPower SOA Appliances, Part 2: Use WebSphere DataPower SOA Appliances extension functions for certificate-based XML standard encryption

Looking for a way to manage the interoperability among applications using
different protocols that need to exchange confidential data? Consider combining the
functionality of IBM WebSphere Enterprise Service Bus and IBM WebSphere DataPower
SOA Appliances. Find out how you can get a secure, agile, and extendible solution
with a little effort in terms of code.

I’d say that if you happen to have both of these products, you are better of attempting to offload most of the logic to the datapower box and use the ESB for business functionality.