Hi Philip Shanks,
Welcome to the world of open source solution selling. We have been selling open source CMS since 2000. During that time most of the enterprises were not aware of "open source" (specifically in California). It was a hard sale.
Now the situation is changed. Most of the enterprises know the benefits of open source CMS. Customers feel comfortable with various aspects of a CMS whether it is open source CMS or an expensive commercial CMS.
#1. Quality of the Product If the quality of the product is good (enterprise ready), then most of the customers do not hesitate to adopt it. For example Alfresco is being used in large enterprises all over the world.
#2. Good Support Services Customers feel comfortable when there is a "stable" company behind an open source software. In case of Alfresco, there is strong company and solid team behind it (please read information at
http://www.alfresco.com) with good support offerings.
#3. Good implementation Customers also feel comfortable with a services company with a good track record of implementing open source CMS and rolling out successfully (please see the customer list at
http://www.cignex.com). I will be able to provide you good material so that you could impress your management with the capabilities of the product and the comfortable roadmap.