Why do DSLRs not use infrared instead?
If you used an infrared sensor to focus the lens, the image would be out of focus in the visible spectrum. The refractive index of a lens depends in part on the wavelength -- that's why a triangular prism breaks white light up into its component colors, and it's also the cause of chromatic aberration. Lens designers correct for chromatic aberration in the visible spectrum by using multiple elements with different materials and coatings, but to correct for aberration in the IR region would surely add a lot of cost and weight to a lens (if it's even possible).
If you look at your lens, you'll see a (usually) white line that indicates the distance at which the lens is currently focussed. You may also see a smaller red line a bit to one side -- this is the infrared focus distance indicator. If you want to take a photo in the IR range, you first focus on the subject. Next, look at the white line to find the focal distance. Finally, adjust the focus so that that same focal distance lines up at the red line instead of the white one.
Here's a Luminous Landscape post that's mostly about taking IR photos with an IR converted body, but which touches a few times (especially at the end) on focusing issues. You may find it interesting.