As someone who had to endure Jane Seymour telling me about life's lessons and her upcoming new fashion line, I find this idea really appealing. Ashland's speakers, though, have proven pretty solid considering it's a tiny liberal arts college in the center of Ohio. Blackwell was good; even Archie Griffin at my brother's commencement was solid.
Still, you need some top-class failures to give graduates a needed dose of reality. My suggestions for really failuretastic commencement speakers:
O.J. Simpson
Scott Weiland (supposedly clean... but meh)
Ryan Leaf
Tim Couch
John Edwards
Bernie Madoff
Adam Jones