Delete Entity by GUID

Hi guys,

I'm new to Drupal API development, but I'm realising more and more every day how powerful it is, and I would like to start using it.

I'm not sure if this is the right place to post questions like this, but here goes:
I'm interfacing my Drupal site with an external system using middleware, OS scripting and Drush. I need to perform a node (entity) delete on instruction from the remote system. I had a look at node_delete, but I don't have the NID of the entity I want to delete - all I have in the instruction message is the GUID which was assigned during creation using the Feeds module. Am I going about this all wrong? Is there a way to get the NID from the GUID, or is there a better way to do this?

Thanks in advance,
Jarus