I think it would require more sensors and a ton of fuzzy logic to even get close to something useful in the sun detection and real-ambient deduction department. IMO, it would be a lot more cost effective and accurate to simply create/use a remote sensor.

I'd think if they had a $50 remote (wifi) sensor unit that measured ambient and exterior temperature they'd sell quite a few of them. It would be a nice extra revenue stream considering a thermostat isn't something a family is going to replace year over year like a mobile phone.