Visible crew/equipment: In the gospel church scene, where James Brown is whipping up the congregation, at one point as Elwood is dancing at the front of the church after Jake has "seen the light", where the camera is pointing down the nave of the church from what would be the altar end, a technician can be seen scurrying, at the back of the church, from the left to the right aisle. (00:19:05)
Visible crew/equipment: When Jake and Elwood walk towards the neo-Nazis and make them jump in the river, a cameraman and two other crewmembers are reflected in the car's window when it starts to run. (00:56:37)
Visible crew/equipment: When they're driving on Lower Wacker Drive in the end, you can see the shadow of the camera. The camera is on the hood of the car. It's visible in the scene where they almost hit a cyclist. (02:08:05)
Visible crew/equipment: In a quick, wide shot of the front of the building at the end of the movie, as the cops are trying to break in, the camera and crew are reflected in the back window of the cop car closest to the camera.
Visible crew/equipment: As Elwood looks up at Mack in the diner, the camera and lighting rigs can be seen in his shades.
Answer: It was a double, and the "making of" documentary on the DVD shows this.
David Mercier
To be fair, John did do a lot of flips and dancing as Jake; just not all of them in the movie.