I've tried two approaches to temperature sensing. For I2C, I used a TMP102 module which is similar to what gnibbler describes. Here's my post Here's my post on that: http://www.element14.com/community/groups/raspberry-pi/blog/2012/07/26/is-it-done-yet-temperature-sensing-with-the-raspberry-pi
For 1-wire, Adafruit recently released there own image, and it contains 1-wire support. I was able to read a DS18B20 1-wire temp sensor with it. More details in this post:
Finally, another approach is to use analog temp sensor and an external ADC. Adafruit has a nice tutorialnice tutorial on this. My rep won't let me post another link but try this learn.adafruit.com/send-raspberry-pi-data-to-cosm/overview