I thought that I read some people were having problems getting certain cameras to connect. Maybe it was user error and not a limitation of the kit? Just wanted to check about that before I made a purchase.
from Apple's support site document, Using the iPad Camera Connection Kit, it says:
"iPad Camera Connector supports Picture Transfer Protocol (PTP) and Mass Storage Device protocol"
AFAIK most if not all recent digital cameras support at least one or the other of these USB transfer methods.
for example if your camera shows up on your computer like an external drive when you connect it via USB, then your camera supports Mass Storage Device protocol, and it can be connected to the iPad with the CCK.
of course if your camera uses SD or SDHC cards, no need to connect the camera directly to the iPad. those Cards can be read directly with the 2nd adapter included in the CCK (but not SDXC - see same support doc for info)
The more recent Nikon DSLRs do not support MTP, so we must use a card reader with the CCK instead of using the cameras mini-usb port to connect to the CCK.