Yeah, I saw your post after I posted the comment. There is one battery grip Nikon makes that does what you're saying - the MB-D10 (D700, D300, D300s) - and it can use the following power:Just a crazy thought, but could Canon produce a camera without battery grip that uses the LP-E6 battery at 7.2V, and have an optional battery grip which uses the LP-E4N battery at 11.1V? That way the people that desire a smaller camera, with a battery interchangable with their 5D3 and 7D are happy, as well as the people that want a faster-focusing camera with the battery grip, higher shots/charge, and interchangability with 1D X. How big would an add-on battery grip need to be in order to accomodate the larger battery? Much larger than a BG-E7 (7D) or BG-E11 (5D Mark III)?
That was my point. I'm pretty sure Nikon even allows that with the same grip, you can either stuff it with regular little ones or a pro large battery. I never understood why Canon didn't allow that. That said it is not something I've looked into much.
(1) EN-EL3(e) - 7.4V 1500mAH
(1) EN-EL4(A) - 11.1V 2500mAH [must use optional BL-3 accessory]
(8) AA batteries - 11.2V (NiMH) or 12V (alkaline/lithium)
However, I don't like that Nikon has different framerates for the difference power sources (especially given that Canon achieves similar results with the lower voltage).