I was reading about the new Asus Android tablet today. It has an optional keyboard, like my ZaggMate, with built in SD card slot, USB port (with HDD support) and additional battery.
If such a device existed for the iPad I would be very interested. Perhaps ZaggMate might take up the idea.
I know that the iPad has an excellent battery life, but the additional battery would be useful to keep you going when the main battery ran out. On the Asus the additional battery functions in parallel with the main battery not as a reserve, but I would like a reserve battery function.