If it's really true that Jon Stewart will host the Academy Awards this year, he'll have the distinction of having less to do with the movies than any Oscars host since Johnny Carson.
The list of hosts shows that the normal practice is to pick a comedian with a strong presence in the movies: Bob Hope and Billy Crystal are of course the most frequent hosts, though Carson is tied with them for most consecutive years as host (four). Note that in the early '70s they went with four co-hosts instead of one host, leading to such weird hosting teams as Helen Hayes, Alan King, Jack Lemmon and Sammy Davis Jr.
The host I would most have liked to see was Jerry Lewis, who was part of a team of co-hosts in 1959 and let the show degenerate into sort of a free-for-all, letting everyone come up onstage at the end. When Dean Martin came up, Lewis said: "And they said Dean and I would never be on the same stage again!" They never let Lewis host again, and the show degenerated into its usual state of numbing predictability, with the occasional streaker to liven things up.