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)