LiveScience reports on David Carrier's current paper in Evolution:
"The old argument was that [apes] retained short legs to help them climb trees that still were an important part of their habitat," said the study author David Carrier, a biologist at the University of Utah. "My argument is that they retained short legs because short legs helped them fight."
I won't be able to get the paper for awhile. The thesis is that short legs are adaptive to aggressive interactions because short, stout bodies are better for fighting. In addition to the primates mentioned in the article (of which there are only nine), Carrier has in previous papers (Pasi and Carrier 2003, Kemp et al. 2005) studied the relation of limb length and fighting ability among dog breeds.
It looks like Carrier is arguing that the biomechanical advantage of short legs in climbing doesn't really predict arboreality:
As indicators of aggression, Carrier looked at the weight difference between males and females and the male-female difference in length of canine teeth, which are used for biting during battle. Studies have shown greater aggression in primate species in which males tipped the scales relative to females.
Primates with the stoutest figures also ranked high on both aggression measurements. For instance, the gibbons boasted longer legs than other apes and also ranked low on the aggression scale. In contrast, male gorillas, which are more than double the size of females, were stout.
The lengthy legs didn't keep gibbons away from canopies either. "Gibbons are the best acrobats in the animal kingdom. There are no other animals that can move through the canopy the way a gibbon can," Carrier told LiveScience. "And they contrast male gorillas, which hardly ever climb. When they do climb they stay close to the trunk, they spend most of the time on the ground."
I don't know -- australopithecines don't have long canines, and their level of dimorphism has been subject to debate. So the comparison may not work. I'll have to see what the paper says.
Pasi BM, Carrier DR. 2003. Functional trade-offs in the limb muscles of dogs selected for running vs. fighting. J Evol Biol 16:324-332.
Kemp TJ, Bachus KN, Nairn JA, Carrier DR. 2005. Functional trade-offs in the limb bones of dogs selected for running versus fighting. J Exp Biol 208:3475-3482.