This really depends on which type of aircraft you're going to photograph & how close you can get to them. Also, what you want the photos for. Unless you're going for huge print sizes even a 3 megapixel camera should give excellent results. A lot will depend on the printer you use.
Unless you live right under the approach or have special access to airfields I would forget the standard 3x zoom digicam. Also forget all about "digital zoom". In my limited experience 10x optical zoom is a minimum requirement for decent shots of aircraft. Check out the Photos forum here to see the excellent results that Rottydaddy & Omag are getting with theirs. You might also like to check out the Fuji S5500 or whatever the latest version is. All the major manufacturers do similar "bridge" cameras (between the standard digicam & DSLR) although I've read that this might not last much longer. Pros & cons of "superzoom" digitals here. http://shutterbug.com/equipmentreviews/amateur_digital_slrs/1205superzoom/
The latest all singing & dancing specs look very tempting but it's possible to pick up brand new earlier models of these cameras at bargain prices if you look around. I took these with my 3 megapixel Fuji S5000. http://www.simviation.com/lair/photos/showpix2.html
PS. It's worth bearing in mind that you will need extra memory & at least one set of rechargeable batteries + charger.
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