Over on the dpreview forums, brandrx found out why a set of NiMH batteries did not last very long. (He uses sets of four batteries for his camera.)
Using a voltmeter, he found that one of the four batteries was nearly dead -- even after a full charge. Upon placing the four batteries in his camera, the one battery affected the entire set, causing the camera to think the batteries were dead after an hour or so.
Replacing the one bad battery with a new one made the quartet work correctly again.
For an overview of battery technologies, read Batteries by KenRockwell.com.