The code and project for the plugin are located here:
This first release contains the plugin source code along with an unmanaged solution that can be installed into Microsoft Dynamics CRM 2011 on-premise or online.
The plugin itself isn't dependant on a specific message, but in the solution is set up so that it triggers on the on-create message of the included currencytrigger entity.
This sort of a setup would be ideal if you wanted to have a timed process that hits the CRM Service to trigger the on-create message to update currencies at set intevals.
-- This sample plugin is based on a web service provided by http://www.webservicex.net/, and I make no claims as to the reliability or validity or safety of this web service. This plugin and included solution is provided as a sample framework for how someone might design and implement a currency updater plugin based on a web service.
-- I make no claims as to the rights of use of the utilized web service (as I couldn't really find any).
--This service appears free and useful, but I make no guarantees.
--You use this plugin and project entirely at your own risk
I hope this helps!!