hbtraveler-
I sympathize!! I am also allergic and if I was in your situation I would ask to be put on another flight, if not possible, then I would insist that the cat/owner be seated as far away from me as possible (preferably the back).
I proposed a question on the thread below, but never got an answer. Could a passenger request that the pet/owner be removed from the plane? Is this within a paying passenger's rights?
http://www.flyertalk.com/forum/Forum50/HTML/013050.html
I realize that there are many pet lovers out there that probably aren't happy with my comments, but please understand that this is not a matter of disliking animals, it is purely a health issue. If I was exposed to a cat on a long flight, I could get VERY sick.
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"I just wanna wish you good luck, we're all counting on you"
-Dr. Rumack