At the high end, intimidating all the other lenses, sits the EF 14mm f/2.8L II USM, a $2,199 prime lens optimized for use with the 1Ds Mark III. Among other things, it's got improved dust-proofing and weatherproofing around the lens mount, and a round aperture (for a more natural depth-of-field blur). Canon says it's sharper than the original in the corners as well as the center, and that the increase to two aspherical and 2 UD elements improves control over chromatic aberration. You'll be able to look through this lens in October.
Additionally, the EF-S 18-55mm f/3.5-5.6 USM gets an image-stabilized buddy in October, the $199 EF-S 18-55mm f/3.5-5.6 IS. A new optical design reportedly improves the stabilizer latitude to four stops. Note that this lens will not use an ultrasonic motor like its sibling.
One more lens makes its debut today, but not in the U.S. We'll probably see the EF-S 55-250mm f/4-5.6 IS here at some point, but for now, you'll have to fly to Europe (or order it online) if you've a yen to do so.
To make the most of your Mark III, Canon has enhanced its Wireless File Transmitter WFT-E2A with a bump up to the WFT-E3A. It supports Live View for remote operation--up to 492 feet--and can act as a USB host for a GPS or mass storage device (to record directly to a hard drive). It will ship at the same time as the 1Ds Mark III in November for $799.
Finally, to complement the new weatherproofing on the 40D, Canon is releasing an enhanced battery grip, the BG-E2N, also weatherproofed. It will be backwardly compatible with the 30D and 20D, and run $270.