Avian – the only one I personally know who designed and made his own motherboard and a brilliant guy in general – wrote on his blog a short article on how to use physics to get your tea temperature to 75°C without using a termometer. Most green (yellow, white et al. of its subclasses) teas are supposed to be infused in water of that temperature.
Á propos, I'll let you in on a more basic trick on how to get your water to 90°C – which is the optimal temperature for most teas other then green.
It's very easy actully – the moment you start seeing small bubbles rising up, you turn off the heat. Because of the impression those small bubbles give, this is some cultures referred to as the pearly boil.
The idea is that at the time the first bubbles start to form and rise, the water nearest the heat source has already reached its boiling point (100°C in normal conditions) , but the heat has not spread allover the water yet. Therefore the water further from the heat source is colder and the average is about 90°C.
Of course, this method doesn't work in microwave ovens and the average temeprature of the water depends slightly on the measurements of the pot you're using – it works best when the water level is roughly the same height as the pot's width.
hook out → somewhat tired, in need of more backup space