OK, when I went to see the film (in it's first week of release), the place was packed, and people were laughing most of the way through. However, 3 or 4 times the place roared with laughter after certain scenes/lines. These were;
1. When Butthead lands in the cockpit and says - "you said c-" followed by the pilot saying "GET OUT!"
2. Out in the desert when Butthead points at the 2 vultures "doin' the wild thing"
3. Anderson going into his camper and saying "pull your damn pants up" to Beavis
- Butthead being thrown out of the window by Chelsea Clinton.
Lots of other stuff too, just these seemed to be the main ones. My favourite part (part, uh huh huh huh) was when Butthead said 'just start running real fast, you'll be OK' when they got out of Muddy's boot (guess you U.S. types call it a trunk)