Evolutionary History of Apes

There are less complicated and tortorous routes which would acheive the same thing. The common ancestor of humans, chimps and gorillas would have been a knuckle walker. Whilst the ancestors of gorillas retained this feature, the ancestors of chimps and humans began to develop bipedality. Some of these bipeds, however, which would include the ancestors of chimps, began to get back down on all fours as their ecology demanded, and whilst they still retained enough of a knuckle-walker's anatomy to not make this prohibitvely unlikely. They'd still have the protusion on the radius, as demonstrated by the fact that it's retained in bipedal Lucy, so there's no need for this to be evolved again.I don't know how realistic this scenario would be based around the anatomy of the fossils we know, but I don't think humans and chimps sharing a bipedal ancestor requires the wrist-locking mechanism to evolve three times independently - it still only needs to arise once. Knuckle-walking as a whole could have evolved once, been lost once, then been subsequently regained once, by an ape still in possession of some of the features that made their ancestors successful knuckle-walkers.

Less tortorous than the same trait evolving independently in three separate lineages, yes. The key point being, though, that it doesn't need to be lost and regained in subsequent populations. It need only be lost once, and regained once.

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Does anyone actually believe that knuckle walking was lost and regained by any of the species?

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I have no idea; I just don't see grounds for dismissing it as ludicrously improbable. As you've pointed out, at least one of the traits essential for knuckle-walking - the wrist locking mechanism, was retained long after hominids had become fully bipedal, so it's not as if everything had to be re-evolved from scratch. And it's not like lost traits aren't regained in some lineages. To reprint one of RAZD's favourite diagrams: Here we see 5 different cases of lineages of wingless insects with winged ancestors regaining wings. Lost ancestral features are sometimes regained, so there's nothing ridiculous about the argument a priori. If our molecular dates are right and bipedality precedes the human / chimpanzee split, it's definitely worth considering.