The answer: According to my unofficial survey and the link below, the Grambling State Marching Band with SIX(!) appearances. Congrats to DJGolf who had Grambling among his guesses. The 2nd most was Up With People (4).
The most appearances by an individual: Al Hirt (3 - he appeared in the first 3 SBs played in New Orleans). Pete Fountain and Carol Channing are tied for second with 2 appearances each.
It is interesting to note that Pop Music Icons in their prime didn't appear in the halftime show until Super Bowl 26 when Gloria Estafan wowed the Hubert Humphrey Metrodome crowd. I was there, she was quite fetching as the pedestal on which she stood rose halfway up to the dome ceiling. It entered a new phase following the 2004 Janet Jackson nipple show by going to "old school" pop and rock performers, past their prime, who presumably will not embarrass the NFL like Jackson did. With Madonna tonight, who knows?
Link to Halftime Performer history, enjoy!:
http://en.wikipedia.org/wiki/List_of...halftime_shows