I realized, thanks to a comment by Hula Buns and an email from the Mormon Child Bride, that I failed to fully explain the rest of my prom dress story.
First, I did get the dress back, and it is residing safely in my closet back at my parents' house. I'm not sure why I'm saving it -- maybe someday I'll get really crafty and one-dress-a-day-it into a cute little cocktail number. Maybe I'll try to force it on my daughter when she's of prom-going age. (Hi, Lola!)
Second, the hideous t-shirt sleeves were not permanent. It was a bolero, or little jacket, that was off-white and did not match the dress. And unlike most hideous boleros which sport cap sleeves, this one had 3-Hanes-to-a-ziploc-bag length sleeves. Ugh.
Now, given that the addition was not permanent, you might be wondering, "Well jeez, what's the big deal?" Which brings me to my third, and really most insulting, point.
When the dress was returned, it was returned with the offensive bolero -- a not-so-subtle hint that perhaps I should think about covering my smutty little shoulders in the future.