Debuging ActiveMerhcant SOAP -> CyberSource

I am trying to use ActiveMerchant to send credit card transactions to CyberSource. I have been trying to follow the documentation, examples, and APIs, but problems persist. ActiveMerchant is sending a SOAP header and envelope to CyberSource with my credentials, but they are being rejected.

To debug this, I would like to observe the actual SOAP that is going down the wire. Is there a tool that will let me do that? I am using Eclipse/AptanaStudio/RadRails with a Mongrel server.