In article <4bb4c5$kh7@lectura.CS.Arizona.EDU>, bmtong@CS.Arizona.EDU says...(most of good post deleted - to add a point to the following:)
>In practice, the camera does not meter ambient light when the flash is
>ready, which is not very convenient. Nevertheless, it is possible to
>manually control TTL fill flash ratio, if you turn off the flash to
>meter ambient before turning it on again.

If you cover the middle of the three F3 TTL pins, the ambient meter stays on when the flash is on (though the camera maximum shutter speed is no longer limited to the synch. speed, and the VF ready light will not work - a good trade for most work, I think).
Hope this helps.