Some supposedly unsupported devices may still work with the iPad, for example some USB sticks and SD cards never before used with a camera. It is possible to then copy image/video files directly from a Mac or PC to the iPad using the storage media, and not just via a connected camera.
I got an Apple iPad Camera Connection Kit for my iPad to transfer photos and movies directly to the device. Unless you have actually used a camera with your SD card, the iPad will complain that the device is not designed to work with it.
The easy workaround is to manually create a folder named 'DCIM' at the root level of the memory card or USB stick in the Finder on your Mac and you are set. Then put all your movies and pics in that folder. The reason this works is because that's the same folder a camera would make and store on the SD card in the first place. Note that this only works with images and video files supported by the Camera Connection Kit.
[crarko adds: I haven't tested this one, but it seems there are caveats.]
Mac OS X Hints