It is nonlinear, but you kind of have to hack it to make it work for vidding. If you're say, starting a project, and you know that the first clip you want to lay down is at 1:27 into your song, well, you can't just drag the clip to the timeline. It will "helpfully" align itself with the beginning of the timeline. You have to create either a gap clip or a placeholder clip of 1:27 in length in order to put your clip where you want.

The good news is that once you have that gap clip down, you can place clips on the track above it any point. So one solution for vidders might be to put down a gap clip the length of your song on the primary track and then vid on the tracks above.