I am (thankfully!) not allergic to anything that I know of, but my DH is allergic to pineapple. Strange, I know -- so is his mother. If they eat something with just a little juice in it, then he needs Benadryl, she needs 911. If he has no Benadryl, off to the hospital he goes, as the anaphlactic shock sets in. I've seen it, after a chinese food lunch - his skin turns red, his neck and face start swelling up, his tongue and throat/tonsils start swelling and cutting off his windpipe. NOT a pretty site!
Luckily, there is no airborne danger to this. I know peanuts have the airborne danger - perhaps if 300 passengers opened candied pineapple bags at once it would be a problem, but that's unlikely.
We turned down an all-expense paid cruise from my parents due to this allergy -- the cruise was a week in Hawaii. We realized that there was simply no way to guarantee that the food we were eating didn't have pineapple, juice, or was prepped on the same surface pineapple had been cut on, etc. So we didn't go.
I also agree that since peanuts are such a 'popular' allergy, letting the airline know about it ahead of time is a duty for the person allergic, and keeping the baggies of peanuts from being servied is a duty of the airline.
No skin off my nose, I prefer the pretzels
