Jay DeSimone , Feb 14, 2012; 09:42 p.m.
With Canon, if you are in Av or Tv mode, the camera will make your flash use ambient light as your primary source, and the flash as fill. If you want the flash to be your primary source of light, you have to use M. I *think* the XT has a flash sync speed of 1/200, so your shutter can't be any faster than that when using a flash.
Since you'll be in M mode, you'll largely be in control of the exposure, although the camera should meter for your aperture (but if it's too wide for your settings, you'll still get an overexposure).
For an educational point, the shutter speed has little control over the amount of light hitting the sensor when using a flash. Think of it this way...when using flash, shutter speed regulates the influence of ambient light, and aperture regulates the influence of the flash. The flash pulse is generally much shorter than the fastest shutter speed you can use with it, so it's up to the aperture to limit the light.
If you aren't using a flash, your point is exactly the way you will need to use your lens. Focus with your lens at it's widest aperture, then stop down to your desired aperture when you're ready to meter and shoot.