I have never been involved in any decision making related to visiting speakers at any of the schools that I attended or any of the business conferences that I have participated in.

The worst (or best, depending on you viewpoint) celebrity speaking engagement that I have heard of was Sasha Baron Cohen at Harvard’s 2004 Class Day ceremonies.

One of the public speakers that I was most impressed with was Jane Goodall. I saw her at the conference for the specialty coffee industry. She has been part of an effort to eliminate the bush meat industry by creating viable economic alternatives to the local people. One of these projects involved growing coffee.

I think that having somebody who is very interesting , but is only tangentially involved in the key interest of the audience is an excellent strategy.

InternationalSpeakers.com has this impressive list up on their site. These are people that they have recently booked for events:

Bill Cosby
Geena Davis
Kevin Nealon
Magic Johnson
Ben Stein
Lynda Carter
Dave Barry
Caroline Rhea
KC & the Sunshine Band
Cuba Gooding Jr.
Neil Armstrong
Goldie Hawn
Lance Armstrong
Naomi Judd

This list is not a menu, they can book virtually anyone for you. I think Rowan Atkinson would be a great guy to have at your graduation. I would like to see the Numa Numa guy doing some keynote addresses for some Web Marketing Conventions. The possibilities are only limited by, well, schedules, budgets, lots of other stuff, but still, you have to think big.