Steviec9 - isn't this essentially the reason for Govt having decided to raise Air Passenger Duty, and thus for short journeys encourage people to go by land rather than flying ?
While a small difference in price means you will continue to fly as it's less hassle, a sufficiently large cost differential between train and plane means you are deciding to take the Eurostar and reduce CO2 emissions (or at least give airlines less incentive to fly a plane on this route)
Separately, I believe new rail lines will open between Brussels and Amsterdam around the end of 2009