I've been using an Arduino, an EtherShield and a DS18B20 temperature sensor to log the temperatures outside my window for about 6 months now, and it has worked well. I've also been getting a couple of emails looking for the source code to my program. The original code was rather quickly hacked together and I was reluctant to publish it (although I did send it to those that emailed).
I got some spare time over the holidays and did a major refactor of the code, including moving from the EtherShield library to the ethercard library. I've published the code to github, and the raw sketch file is available here.