I left NYC for a powwow in Minnesota with my 5DIII and trepidation over having one 32Gb CF card and one battery. I even stopped off at a camera shop in Minneapolis and found that they carried the Canon battery but the price was so high compared to NYC prices that I didn't get one. I did have a charger and a laptop with a portable hard drive to dump each day's shots onto.
The first day I was somewhat conservative in the number of shots I took (CRW's only), but I did get about 800 on the card with room left over. The battery had about 15% left, even after lots of chimping.
Second day I decided to shoot with complete abandon. By the end of the day I could not fill the CF card until I shot video after video, just to fill the card. The battery still showed about 8%. I shot more, including videos and HDR sequences, than I ever had in previous years with the 1DII and came to the conclusion that, while it is probably good to have a backup CF card and battery, there is just about no way that I would run out of either in even a heavier than normal day's shooting for me. (Minnesota at 30 below zero is a different story...)
By the way, I am a complete fan of the 5DIII now. What a nice camera!