I suspect it has got to do with either one of the following reasons:
1. Driver issue
2. Internet Explorer issue
3. Flash issue
Until there can be a more clear solution. I wouldn't want to risk any of my viewers missing out on the fullscreen mode or telling me their screens locked up. Over the next few days, I will change all my VRs to the version where "Change state quality" and "Change Bitmap filtering" are turned on. With those options on, the fullscreen problem doesn't exist.
I will also take the oppurtunity to update to hum@no.id's 'Reflection' - nearly 3D skin