The Camera Connection Kit provides you with two adapters - one for SD Memory Cards and the other for USB.
There are a myriad of other memory card formats, including the Compact Flash Card used in many of Canon's higher end cameras. The iPad CCK will not allow you to move photos from a CF to the iPad.
I just obtained a Delkin Devices Reader-38. See here for details:
Delkin Devices UDMA-enabled High Speed USB 2.0 multi-card reader – All-in-One reader/writer
It will read 18 different memory card formats and send them through a USB cable to the iPad CCK USB adapter. Delkin's Customer Service did not know whether it would work with the iPad.
It does! I just tested it, and it works very nicely. Now we can input a whole bunch of additional memory card formats!