No image on TV when connected using HDMI cable to MacBook pro when booted in bootcamp
I got it to work, I'm not sure exactly what I did in what order, but based on my Firefox download history, I think this is what I did.

First I downloaded and intalled version 285.62 (285.62-notebook-win7-winvista-64bit-international-whql.exe). This version gave me the symptoms described in the question.

I then uninstalled all NVidia drivers, which I assumed rolled the driver back to the Bootcamp driver version.

I then downloaded and installed driver verion 260.99 (260.99_notebook_winvista_win7_64bit_international_whql.exe). Now the HDMI output works in my Bootcamp installation.