UPDATE:
The Just Tires guys replaced the stud without any hassle whatsoever last week. It took a couple of days to get the actual stud (it was backordered) but I commend them in the customer satisfaction department.
Anyways, yesterday I figure I'd just check to make sure the lugnuts were tight - just to be on the safe side. I pull my center cap off and ANOTHER LUG NUT FALLS TO THE GROUND!!

It wasn't the new one, but I think it was the same one that was off the last time. I'm almost positive that the Just Tires guy tightened it properly - I was standing right beside him when he did. All the other lug nuts were tight too.
I put it back on myself and I'm going to check it in a couple of days, just to make sure that the Just Tires guy put it on tightly - I know it is on properly now. I'm going to be really pissed if it becomes loose again.
Anybody have any idea why a lug nut would loosen and just fall off like that?