For this particular one, I think a physical button would be better, personally. I know it's totally MySpace scene, but many of the camera phone pics I take involve me holding the camera out in front of me and taking a picture of me/us at arms length. Granted not everyone uses camera phones that way, but it is one of the more common ways they get used. And wrapping your fingers around to be able to touch the far side of the phone is inconvenient for that. I think the volume button would be a lot better, also.
EDIT: It'd be natural if they zoomed, yes, but it seems a little senseless in the absence of a zoom lens, which probably isn't coming soon.