When I was a new member, two girls in our chapter were the current and immediate past Sigma Chi Sweethearts and they both had letter sweatshirts with "Sweetheart" embroidered below the letters.
Then for a couple of years, their sweethearts didn't have sweatshirts. When my roommie was named the new sweetheart in March 2005, I got her the same sort of sweatshirt - Sigma Chi letters in their colors with "Sweetheart" embroidered in gold. She loves it, the guys love it, and she wears it all the time. (She was nervous about it at first, but asked their president if it was okay.)
Their view is that if they've picked her to be a female representing their chapter, she may as well respresent.
I'm glad this thread got bumped!