Katie being tickled
I saw the show... it must have been ten years ago.
The TODAY show hosted a company that was seeling one of those waterjet massage tables (they've been on display at malls across the county). Katie kicked off her shoes, climbed in, and the company rep closed the top.
When the waterjets engaged and made their first pass over Katie's soles, she squealed "It tickles my feet!" This went on for the better part of a minute. It wasn't the most spectacular tickling in mainstream history, but nonetheless enjoyable, and I'm sorry nobody taped it.