The iPad is not really a viable backup tool. It doesn't really have the tools or a lot of space. Among other things, deleting the images you've loaded is a one-by-one event. The load process itself using Apple's camera connection kit is excruciatingly slow, especially if you're shooting Raw. And it's difficult (although possible) to manipulate the with Raw images on the iPad. I use it to review selected images on the road and either blog or email them with light edits. To this end, I shoot Raw+JPEG, use the jpegs on the iPad, and then the Raws at home.
The best backup I've found is a camera with dual card slots, like the higher-end Nikons. Sadly, my m43 cameras don't offer this so I've adopted the Hyperdrive. Like the post above, I bought the empty case and added a separately-purchased 1TB drive. It's small enough (the size of a thick large smartphone), light enough, and has a crumby LCD that lets you verify that the copy actually occurred. I backup daily and keep the files on the SD card until I return home.
If anyone else has a better idea, I'd love to hear it.