Same problem here. I've made measuring system with ATMEL's ATMEGA8515 and 12 DS18s20 and on room temperature 8 of them show difference of less than 0.3 celsius but 4 are way off. One actually shows 4 degrees lesser temperature (instead of 23 it is 19). Have no solution for this except to throw those "faulty" sensors ( my goal is to make measuring system with 110 sensors, so i will probabbly have to obtain 200 sensors to make this work as it sould).
Setup: ATMEGA8515, normal(not parasit) power for DS1820, distance among DS approx. 3 mm. Of course, done all CRC cheking, so it is not a commuication issue, but internal hardwer of DS1820.