I’ve recently been trying to get a DS18B20 working with the Raspberry Pi, and although its well documented around the internet I ran into a bit of an issue where mine was always returning t=85000. Although alot of people claims this is wiring (which could be the case) for me it was actually to do with how it was reading the value in the kernel module. Originally in modules I had:

w1-gpio w1-therm

As it turned out, there’s a problem with the pullup (http://www.raspberrypi.org/forum/viewtopic.php?f=37&t=48588) where the below resolved the problem:

w1-gpio pullup=1 w1-therm