Canon has a high-speed sync technology on some of their cameras, where the flash strobes multiple times while the shutter moves across the film.
The XT has a focal plane shutter, like almost every other SLR, as far as I know. FP shutters have two curtains, and when you exceed the sync speed the closing curtain has to start moving before the front curtain gets all the way across the film opening/digital sensor.
The max sync speed is the highest speed where the second shutter doesn't start until after the first shutter gets all the way across, so the entire sensor is exposed when the flash goes off. Making the curtains move faster is more expensive.
For what you say you want to do, have you considering using a neutral density filter?