Macbook Pro (March 2008) Trackpad Preferences - multitouch

I keep catching the trackpad on my new Macbook Pro, usually when I'm in Safari, and it zooms the text - I assume I'm accidently doing a two-finger 'pinch', as a reverse pinch puts it back.

Does anyone know why there isn't a checkbox in System Preferences / Keyboard & Mouse / Trackpad next to the 'Pinch Open & Close' - I guess if there was I could turn off this option?

Is there any other way to switch off the 'pinch' to zoom option?

Thanks very much!