the question

http://www.grist.org/advice/ask/2008/07/09/index.html

This question isn't relevant. But what is is a comment towards the bottom that asks:
"if the plane is flying anyway, a commercial flight on a regular schedule with an empty seat, then isn't it less carbon to hop on and not drive?"

The rebuttal is of course: "While I certainly see what you're saying, this is basically a twist on the argument, "Well, the product has already been made and is in the store - I can't help the production impact, so I may as well buy it!"
If fewer people fly a given route, choosing instead to take alternate transport X, then the regularly scheduled commercial flight will stop being run, or will run less often."

This is a question I will be grappling with over the next few months as I begin to travel more.