You can, but you would have to move around the vertices at the neck so they met up correctly with the new torso mesh. Then, depending on what you're doing, you would have to reweight the whole model.
If you're taking the head off of Kyle Katarn and putting it on Luke's body (for example) you would have to weight the whole body again.
If you're taking a head off of a model and putting it on a model you have that's already weighted you'd just have to weight the head (I think).
Hope that helps.