The current issue of IEEE Spectrum has an article on the current state of goal-line technology and the chances of a system being adopted by FIFA in the next two years. Nine companies will participate in FIFA's first round of testing in December, and a select group will be invited for a further round of tests in the spring of 2012. FIFA could then choose a winner or decide not to proceed further, and given Sepp Blatter's and Michel Platini's pronouncements on video technology the second option is a strong possibility.
The technical challenges according to FIFA are to (1) detect 100% of all ball movements toward goal under all conditions, (2) inform the referee of a goal within one second, and (3) be affordable. It's the classic "good, fast, cheap — choose any two" problem; the first challenge will be the most demanding and will influence the ability to meet the third. Such a system has to work under occlusions, day and night, under all weather conditions, and the camera has to have a high-enough bandwidth to track the fastest-moving balls. Hawk-eye has developed a goal-line system that has been tested in the Premier League; Cairos-Adidas and Chronolec-Tag Heuer have developed systems based on a sensor in the ball and a magnetic field generated by embedded wires in the field. I have to think that FIFA would look very dimly on anything intrusive for goal detection, but the speculation is that these are some of the companies being considered. (The true list is being kept secret by FIFA.)
The technical challenges are not as simple as some people think when they look at the use of technology in other sports, but I believe that the biggest challenge to goal technology is cultural.