From our own celeb list:
BRITNEY SPEARS:from TMF member and celebrity pedicurist Serialtickler comes this golden nugget of information: Britney is really ticklish right under her toes, she almost fell outta the chair when i "acidentally" tickled her their. I told her sorry and she told me it always happens during pedicures. ALSO: this was from the Ironic Tickler’s website: Britney Spears: Oh baby baby! Britney Spears, that hot singing sensation, is ticklish on her sides and the back of her legs. This comes from a secret friend. Friends are cool. BONUS: She also admits in a magazine article that she used to get tickled by her brothers when she would get mad at them.
But maybe the most convincing evidence is from a Making the Video special where it appears that one of her dancers gives her a very quick tickle to her side, causing her to squeal, giggle, squrim and flinch away.