I managed to resolve this myself. The following article explains how to set up custom resolutions using Regedit:
Set up 1440x900. Works like a dream.
- AMD need to formally publish this as a "how to"
- AMD should consider adding 1440x900 as a standard resolution in the BootCamp drivers.
With the crimson drivers, you can setup custom resolutions on the additional setup/info popup. Prior to crimson, I was using the registry to setup a resolution.