In all my (almost) two years on the board, I have never been able to figure out what YMMV stands for. I figured out all those other pesky abbreviations - even the suspicious-looking IANAL, but YMMV continues to elude me. Please, someone help, I am desperate!
Your Mileage May Vary: my experience has led me to this conclusion, yours may lead you to a different one.
AFAIK, IIRC, YMMV = Your mileage may vary" - i.e. you may have a different opinion on the matter.
I had to ask a doper in the flesh to find out.
I just have to add that the first time I encountered the acronym was on this board, in a thread engendering some very heated discussion – and my initial impression was that it stood for “You Make Me Vomit” 
P.S. I also included this information in the “technical issues FAQ” sticky thread you can see in this forum.
Ah, that explains it. Thanks.
(I figured it was something along those lines … but I could never get what the first M was for … mileage, eh?)
Yeah, “mileage”, because YKMV just doesn’t have the same ring to it…