Why do some batteries have three terminals?
I couldn't find the information on dvinfo. The best answer I could come up with is that the third terminal is used for information of some sort.

I bought two 8000mah batteries to use with my aiptek go-hd when I go on a bicycling trip. Just curious as to what I may be losing by not using all three terminals. From what I can tell when I blocked the middle terminal I won't be missing anything.

some high energy (like li-ion) have a pin for battery status.
can be only a thermo-resistor or full serial data from a processor.
some other have a pin for charging (that is not protected by a thermofuse).

Some batteries may use the third terminal for monitoring the temperature of the battery during charging.

Others can be much more sophisticated, as Giroud has properly stated. Sony InfoLithum batteries fall into the "much more sophisticated" category.