If you live in a remote area, it can be difficult to avoid air travel, however for many people, air travel is largely discretionary.
Airplanes produce at least as much greenhouse gas emissions per kilometer travelled compared to travel in a medium sized car. However if you travel in Business Class or First Class, the emissions can be 2-3 times the equivalent emissions from a car. The problem is then exacerbated by the huges distances travelled by plane.
Most frequent flyers will find that the emissions associated with their flights far exceed the other emissions for their household. In my previous job with BHP Billiton, I travelled 363,000km during 2006 - all in either Business Class or First Class. This represented 90% of my overall emissions.
The key with air travel is to reduce it wherever possible.
For example, if you travel for work to participate in a meeting, look at opportunties to participate in the meeting via teleconference or videoconference.
If your travel is for a holiday, think seriously about whether or not you could travel somewhere closer to home.
If your travel is less than 1000km, then give consideration to travelling by bus or by train. While it is true that this will be slower, you will produce less than 10% of the emissions of flying, and you will have the added benefit of being able to take more gear with you than is feasible when flying.