When I travel, my motto is: make space, don't erase. At first this may sound insane to you. But as you plan your trips, try to calculate how much camera memory you'll need, then get your hands on enough of it to cover your estimated shooting. Why?
Because you always want to have your pictures live in two places. Many photographers upload their images to a laptop in the evening, then erase their cards in preparation for the next day. In that scenario, your images only live in one place, and that's on a computer hard drive. If you were to keep your photos on the memory card too, then you would be much better protected against mishap.
I also recommend printing your contact information on each memory card in case it is misplaced. Plus, if you leave your camera behind, the card that's in it will tell people how to find you.
Currently, I'm packing a variety of 16GB memory, including the Kingston Elite Pro 16 GB 133x CF Cards
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Surely you should just take a usb powered hard drive with you. Onto laptop, backup to usb drive and done! Lot cheaper than memory, esp when shooting a 3 day festival on a 5D2!
Actually, I do take an external hard drive too. It stays with my computer, and the memory cards stay with me.
There are limits to this, as you pointed out. But, most people are not shooting a 3-day festival with a 5D Mark II :)
Derrick I agree with you on having pictures living in two places. I use the double slots of my D300s where I have the CF card as primary and the SD card as back-up. This works great for me and I don't have to bring the laptop with me. Maibe the iPAD a possible solution too.
I take this advice even at home -- I rotate several 16GB CF cards, putting the latest one on the bottom of the stack and taking the top one for the camera. That way, the images on my Mac are never the only copies, even if I don't run a backup for an hour or a day.