You might eventually replace a sun sensor by...no sensor at all. I am using a combination of WeatherUnderground plugin (exterior temperature and sky condition) ...
true .. that does work as well,
as i did pretty much exactly this for about 2 years.
but here in Thailand you can't even trust the local weather forecast longer than 10 minutes
also the weather conditions are extremly localised .. (seriously ... it can rain here and 200 meter down the road is not a drop in sight and dry streets)
the next weather undergound station is about 10km away .. too far for this margin of error.
RichardTSchaefer's idea with a temperature sensor working out rather perfect, as the direct sunlight does heat up the glass (and the sensor directly behind it) within secounds.. i have a trigger value set to 45degrees Celsius to "detect" direct sunlight ... works like a charm and almost realtime too
the sensor is positioned on the most north corner of the window, so it will cool down almost instantly if the sun passed by.
the vera then ajusts Airconditioning setings (fan direction and +1degree) and re-opens the curtains ..
worth the 2,50 USD

(well okay plus another 5USD for 10 meter telephone wire

)