Don't refer to it as a fetish.
I'm pretty much in agreement with 'Chief and TTD. It all depends on whom I am telling. If it is a male friend, a comrade, I would probably just say something like "I totally dig it when a woman tickles me."
If the person I'm telling is a female friend, then I would play it by ear, depending on the situation. The topic can be approached from any number of directions. If you are already talking about fetishes and/or kinks, I'd definitely mention that I think tickling is very erotic. If you're talking about political science, it would be rather difficult to work in the tickling thing.
Here is an example of how I let one female friend know about my tickling fetish. We'll call her "Melanie" for the sake of anonymity. Melanie is a very attractive outgoing woman in her late 40's. She is unusually well preserved, and the object of much oggling around the office. She's always in a good mood and a joy to be around. Sometimes she and I would sneak out to my car to smoke a joint or a bowl. On one such occasion she noticed my bag full of magazines.
"What're those?" she asked
"Oh, you don't want to look in there, Melanie"
"Why not??" She said as she pulled a stack of tickling magazines out of the bag. "Oh my!"
"Definitely not your typical waiting room magazines, huh?" I quipped, as she thumbed through Tied & Tickled.
"Is this really what you like" she asked.
"For as long as I can remember."
We didn't dwell on the subject, and though I could tell it was not exactly her cup of tea, she nonetheless was totally cool with it. No, it didn't launch me into a new tickling relationship, but neither did it affect our current friendship in any way. I'm happy knowing that she knows and accepts this about me, even if she doesn't necessarily indulge me. After all, she may one day decide otherwise. 🙂