pvq- Are you talking about batteries for the Prodigy or the Protege?
The Prodigy CAN use 18650 sized batteries which have a 4.2 max voltage, and (so far) a maximum capacity of 3000mah. However, these cells are twice as big as the RCR123a or 16340s, which limits max voltage to 4.2V. As production catches up, Steve will be releasing different switches with and without (different) resistors giving voltage options of 3.7V (regulated) 5V (regulated) and 4.2-8.4V (unregulated)
The Protege uses a completely different cell, the 14500, which is the same size as AAs. The biggest capacity of this cell that I've seen is 900mah. Also the configuration of the protege is for a single cell, thus the limitation to 4.2 volts which i assume will be regulated to 3.7 via resistor.