There's a quick blurb in ScienceNOW about a study showing that great ape tots laugh when tickled--hey, just like us! And for all the cuteoverload-lovers (Dana), the story also has footage of an orangutan being tickled.
In the study, researchers tickled baby apes and recorded their responses. The acoustics of the laughs were then compared to humans-being-tickled laughs. The results indicate that tickle-induced laughter in humans, chimpanzees, gorillas, and orangutans is homologous, that is it has the same evolutionary origin. [A siamang (essentially a large gibbon) was also included in the study, and while its response to tickling was similar to the orangutans', it somehow did not display full-fledged laughter]. Nevertheless, the quite vocal human laugh is distinct from the apes' laugh. Quite probably the difference in vocalization is related to the fact that humans speak and apes don't.
In the ScienceNOW states that the study "challenges the notion that exhaled and voiced laughter are 'a uniquely human trait'" (me quoting the commentary, which quotes the lead author of the study). I immediately cringe when I hear/read the phrase "challenge(s) the notion," or often, "widely-held notion." How trite. But I do think it is an interesting study (not like the one wherein they fed cooked foods to great apes...). For starters, researchers can begin to look into why exactly the acoustic patterns differ between humans and apes.
But more importantly, it opens the issue of why humans and animals laugh when tickled. Being tickled is an interesting feeling, because even though it makes you laugh, it also makes you uncomfortable and vulnerable--no one when tickled exclaims, "yes, keep tickling me!" (I've even heard urban legends of people wetting themselves when tickled). Unless tickle-laughter is a secondary result of selection on other things, I would imagine that the phenomenon evolved because of its value in social interactions--interactions which the study suggests are as old as the great apes.