As someone who has suffered from a nut allergy my whole life (now late 30's) I strongly believe that nuts should be banned on flights. I am alarmed at the ignorance of some posters on here, who only think of their own enjoyment and not the safety and wellbeing of others.
My allergy is not severe, I eat foods that 'may contain nuts' as most food items now state that as precaution. I have been hospitalised on a couple of occasions but usually anti-histamines are good enough. My face puffs up, eyes close, itchy throat, vomiting and I cannot breath, its scary and uncomfortable. I have had people eat peanuts at lunchtime and kiss me later in the evening (quick peck on lips or cheek) and my face and lips have swollen up.
I was not aware and had not experienced to my knowledge an airborne allergic reaction to nuts until a couple of years ago at Christmas when I visited a family member who had a bowl of nuts on the coffee table, along with the other usual Xmas treats. After a short time being in the room I began to get all my normal indicators that I was having a reaction. I started to have difficulty breathing, and had to leave their house and take continued doses of medication to control the asthma attack that I was now having - Yes this was because of the nuts and yes it does happen, airborne nut particles and subsequent allergies are real!! For someone with a severe allergy it can be fatal. I was able to leave that environment before anything serious happened but in the enclosed environment of a plane, with recirculated air that is not possible.
Everyone should be able to travel safely and if there are known risks then everything that can be done should be done to remove that risk :)