Hi Nick,
Do you have a crossdomain file on the root of your web server ? To access data from a different server other than the one hosting your Flex application, the remote server needs to have a cross-domain file in the root directory. If you are using ArcGIS Server .NET , add a crossdomain.xml to your web server's root directory, for example, C:\inetpub\wwwroot.
More information can be found at the link below:
http://help.arcgis.com/en/webapi/flex/help/index.html#/Using_crossdomain_xml/017p0000001w000000/