For those running into problems with limited iPad capacity, or anyone so paranoid (like me) that they want three copies of all photos taken:
1) Transfer RAW images to iPad using Camera Connection Kit
2) Jailbreak the iPad and install iFile, which is a free file browser for Jailbroken iOS.
3) Within iFile, copy RAW image files *from* the iPad *to* another SD card - which becomes the third copy.
4) (optionally) use iPhoto to rank/flag photos. Photos that have been ranked of flagged in this way remain on the iPad, in the form of reduced resolution jpegs, even after deleting the photos from the Photos App. These are useful for viewing your favorite photos during the shoot, but are obviously of no use for subsequent editing.
5) (optionally) delete photos form the Photos App to make room for more, if you hit the iPad's capacity limit.
If you only do (1) to (4) then you have three copies (assuming you do not delete the card that came from the camera), the SD card from the camera, the iPad, and the SD card used in (3).
iFile has lots of other advantages too. For instance, you can store movies on an SD card and view them (provided they are in iPad compatible MP4 format) without transferring them to the iPad.
Advantages: three copies for the paranoid; spare SD cards *much* cheaper than upgraded iPad.
Disadvantages: have to jailbreak (but this has many other advantages associated with it ...); need to know where iOS keeps the RAW image files in the guts of its filesystem.