How to centre a 3D pinch zoom

A standard 2D pinch zoom centres the zoom on the object between the two touches. That is, whatever is at the point midway between the two fingers stays there when zooming in or out. I have implemented a pinch zoom by following the very helpful Wiki. It uses a translation along Vector3.forward, so the zoom is always centred on the middle of the screen. I have tried to displace the camera as it is translated forwards, but always with odd results.

Has anyone got a general solution: how can I centre a 3D zoom on what is between the user's two fingers?

(See this blog entry for a 2D solution: http://adtsai.blogspot.co.uk/2010/09/pinch-zooming-using-xna4-on-wp7-getting.html)