Because the question keeps coming up I've shot a set of samples using a Tamron 18-270 VC on a Canon 1Ds mark 2. I've also shot a pair of photos (they aren't great) to show that the lens mount is similar to a Canon EF mount rather than the Canon EF-S mount. I've owned the Sigma 18-200 & 18-200 OS & they also had a rear mount area similar to the EF mount & can be mounted & used on full frame but with similar results image wise. PLEASE NOTE this is just for informative purposes & I am NOT saying you can or should use lenses designed for crop bodies on a full frame body because I have only done it with the equipment mentioned here & do not have access to all the other possibilities out there. Canon brand EF-S lenses extend further into the mirror box area & ARE NOT usable of full frame bodies without potential problems, and some could be serious.
At 18 mm
At 35 mm (barrel markings proved to be pretty accurate)
The lens mounts compared, Tamron 18-270 VC (left side) & Canon 24-105 L IS You can easily see why the lens designed for a smaller sensor can't direct light to all of the much larger sensor used in the full frame body.