I always refer to other peoples children by the name they were introduced to me as, or ask the parents (or child) what they prefer.
My DS1 is always called by his full name, DS2 is often called by a contraction of his name, but sometimes the full name. I always introduce them both using their full names though, because I like them (obviously lol).
In answer to your question, I dislike people shortening DS1's name (because I don't like any of the contractions), but don't mind people shortening DS2's.
ETA I don't like DS2's name shortened to any but the version we use. I'm fussy. If anyone uses a version of my sons' names that I don't like, I just tell them outright 'please call him ...'. I've given up being subtle. Some people just don't get it.
[Edited on 14/02/2010]