when in doubt, call and ASK. My sister just ended up in the ER over her peanut allergy this weekend because she assumed because she ate at this restaurant many times in the past, that she was 'safe' to eat there. well, they changed the recipe on one item.

translate this to airlines... just because we do not see them on our flights, doesn't mean your flight will be peanut free.. always ask the source.
that said, here is what i found:
http://www.continental.com/web/en-US/content/travel/inflight/dining/special/default.aspx
Continental states: "Continental is committed to the safety of its customers, including customers with peanut allergies. We do not serve packaged peanut snacks on our flights; however, some in-cabin food offerings may still contain nuts or trace amounts of nut ingredients, including peanuts. Additionally, other customers may bring peanuts on board. Accordingly, we cannot guarantee the aircraft and food offerings of Continental will be 100% peanut-free. If you have health concerns, we strongly encourage you to take all necessary medical precautions to prepare for the possibility of exposure during flight"
United, which is actually the same company as continental states: "Peanut information
United does not serve peanuts as snacks or use peanuts or peanut oils in foods served on our flights. However, we do serve vendor products manufactured in facilities that also produce items containing peanuts or peanut oils, and we do have snack mixes that contain other tree nuts, such as almonds and pistachios. Since we cannot prevent customers from bringing onboard their own peanuts or food items containing peanut products, we cannot guarantee a nut-free environment. When making your travel arrangements, we recommend that you assess the extent of any allergies or other medical conditions to determine if air travel is the right choice for you."
http://www.united.com/page/middlepage/0,6998,1057,00.html
delta does not have easy to find info on peanuts, here is a contact page:
http://www.delta.com/emailus/servlet/EmailUs?cmd=go
u.s. air is indeed vague, i think they do not serve peanuts alone, but didn't want to state that and have it interpreted that they don't serve peanuts at all, and end up with a meal box with peanut in it.

here is there contact page, to get a better/straight answer:
http://www.usairways.com/en-US/contact/general.html
and finally airtran doesn't state anything i could find about peanuts. i would contact them:
http://www.airtranairways.com/contact/contact_us.aspx?nav_id=91
good luck and stay as safe as possible!
p.s. my sister was okay in the end. they ended up having to give her a steroid...

and she has to take it for a few days, because it was such a high dose. but still, at least she is okay.