Amazing how subjective this is. Many people have responded that they never get too hot on a plane. My experience is just the opposite.

On a recent MSP-NRT flight, I was wearing really lightweight travel pants, a cotton t-shirt, and a lightweight travel shirt. To give you an idea about the weight of the travel shirt and pants, I wore them frequently in Singapore and was generally as comfortable as I was when wearing shorts. A couple hours into the flight, as others were adding layers and snuggling under blankets, I got so hot I had to go to the lav to peel off the t-shirt. And I was sitting in an exit row.

I agree with the advice of others, to pack enough to add or reomve as your comfort dictates. If you're one of those people like many on this thread who get too cold, you should generally bring warmer layers (think sweatshirts). If you're like me and you more often get too hot, your layers should be lightweight stuff like coolmax. I find that the lightweight travel clothes sold by Ex Officio do very nicely on a plane.