Not a totally new tutorial but still a favourite topic! I delivered this to a very technical audience at DPC and really enjoyed it!

http://www.slideshare.net/lornajane/web-services-13234948

I also prepared a bit of example code (look out for some bugs in the rest bits - pull requests very gratefully received!) which you can find on bitbucket: https://bitbucket.org/lornajane/web-services-tutorial-code