I think asking someone to marry you at a sporting event is overdone (besides being a bad idea for other reasons). Nonetheless, I just saw someone on the Pirates telecast holding up a sign for the camera asking a girl to the prom, and for whatever reason, I thought that was really cute.
EDIT--Update, the announcers just said that the girl said yes, lol. How did they find that out? I assumed that since he was holding up a sign that the girl was at home watching... *shrugs*
...I guess in the age of cell phones things happen fast!