I watched a pretty useful tutorial by Joe Simon about using a zoom lens on a glidecam to change focal length without having to rebalancing.

I used to use a Tokina 11-16mm but switched to a Canon 17-55mm and I must say, it does indeed give your footage a completely different look. In Joe Simon's video, he used a 5d and a 24-70mm but my 7d and 17-55mm is about the equivelent to it.