Yeah, I've known this for a while, but I've always hated the term. I never got into MUDs. So, when I started playing EQ and all of these people were calling the monsters "MOBs", I got really confused. When it was explained to me, I said, "But those aren't the same things at all...a MOB is a programming term in MUDs, it's a way to use objects to program a monster. It shouldn't refer to the monster itself."
But, it was so wide spread that it is pretty much now the most common way to refer to monsters in games. Even though it appears nearly 90% of the people who use it don't even know what it means. It may be technically true for all MMORPGs, but it isn't a true or accurate term for monsters in D&D games. Which is why it annoys me.