<<I may be off the mark here but during my ATPL ground school i was told that the Americans can turn off the GPS system at any time they so wish (for security reason etc>>
The same thing was told for me during my ground school as well. Despite the fact that Clinton signed a contract earlier that the precision of the GPS will not be manipulated by Americans in any circumstances we can't rely here in Europe only for that. A system which could be manipulated by Americans "just by pushing a button" can't be the main precision approach system for aircrafts.
For that reason I believe we will not see GLS widely used in Europe for a long time. At the moment Europeans are planning to have the Galileo system operational in 2008, which would be a satellite system fully operated by Europeans. However, it might take still many years before the Galileo has enough coverage for GLS operations.
Due the these security reasons other regions might develop their own satellite systems as well.
Best Regards,
AM