Reset iPhone Safari viewport after custom zoom

Question

Does anyone know how I can get the iPhone Safari viewport to reset on the scale of my choice: does the user scale using a pinch? It seems that although I can use jQuery to zoom when I like:

 $('meta[name=viewport]').attr('content', 'width=640, maximum-scale=0.5'); 

Once the user is scaled, is there no way to zoom?

Ideally, I should let the user scale, so disabling user scaling is the last resort. It seems like this is impossible. I considered using jQuery to reload a page, but this is not ideal, but it will now be checked and reported.

Thanks Tom