(I know it has been asked before, but I thought I'd check in again..)
Has anyone had any success implementing the Dallas one-wire protocol on the NRF51822 - in particular with the DS18B20 temperature sensor?
Yes. The guts of a working implementation are here. (This is a work in progress; some of the comments are known to be wrong, but the code functions and the temperature and serial number are correct.)
An updated version is part of my bspacm package. AFAIK it still works; if there are bugs I may be able to provide a patch, though my own development is in (unreleased) C++14 code.