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.