Maybe should have mentioned that this was an Air app in the original post. Finally found a solution, not sure if it was the most elegant solution. After attempting the web and http service route (which always resulted in 401,403 and some other errors that were probably related to authentication) I went back to my original URLLoader code. The issue I was having was that the returned data was HTML (with no free/busy information) and not the XML I wanted. The missing piece was found in a blog post by Henning Krause...
It is important that you send that User-Agent string along. If you don't pretend to be an Internet Explorer 6 or later, you won't get an XML response because OWA thinks you are using a down-level browser which don't understands XML.
So after adding a userAgent string to my URLRequest I was finally receiving the XML and the free/busy information from Exchange.
RTC of Southern Nevada