I think the Nikon D70 is an epic choice and particularly in the next few weeks. Why? It is being replaced by a new Nikon tomorrow. Fabulous camera, but you will need a biggish zoom for airshows. At least 300mm and preferably 400 to 500. These lenses are fairly expensive, but will form the heart of your system - NOT the camera body :-)
You have many more options now than a year ago and in a sense, it is more confusing. If you are starting in digital SLR cameras, the Nikon D50 is a tremendous bargain and provides superb quality. I used to use a D70 but upgraded to the Nikon D200. I cannot honestly recommend this camera at this time as it has given me nothing but grief since purchase and until this issue is resolved would rather keep quiet, (2nd body, too) although for airshows, it should easily outshine the D70. The Canon EOS30D is a great bargain right now, and if you get either Nikon or Canon, then I am sure that you would be delighted.
Physical handling, is something very important. If it feels right and feels good, then you are in a better position all around. If you for instance, sat a Nikon D50/70 next to a Canon EOS350D I would be confident which you would put down first.
There is so much that I could say here, but the best bet, is to ask the question and I and others will do our best to answer it. It can all become rather expensive, as the camera body is just the start point. Apart from lenses there are memory cards and better software to process your pictures, plus all sorts of other little add ons that will keep your "Oblivion Express" card in regular use. Let us know what you have in mind other than airshows and we can try to help a little more.
My own D70 airshow shots can be found in my Photodump here. There are some goodies, some average and a few howlers. It is just a repository, but you are most welcome to have a look around.
http://s5.photobucket.com/albums/y192/zorbathegeek/
Conan