Unfortunately, you can't shoot tethered with the 1Ds Mark III.
The list of Nikon and Canon cameras that support tethered shooting is here:http://support.apple.com/kb/HT1085?viewlocale=en_US
Aperture is using a standard tethering protocol (I forget what it's called) and the newest Canon cameras don't support it (the older ones do, go figure). Don't know if this is something Canon will fix/address or if Aperture will be adding support for them via a totally different method (or if it's coming at all).
Thanks - I'll keep using the wireless transmitter, works great! I wanted the wired/Aperture
as a backup.