OK, I found the datasheet on the dht22, I wasn't thinking and hadn't tried searching for it. It appears it's +/- 0.2C. I've also been unable to find humidity sensors with better than +/- 2% RH accuracy (perhaps the tech just doesn't exist). After doing some more research, it appears a 0.1% accurate thermistor could approach 0.1C accuracy on the arduino, the big limitation being the arduino's ADC. I haven't been able to find dedicated arduino thermistor reading boards.