Most dSLR's come with a kitlens, but the range isn't big enough to do aviation photography. It's usually in the 18 - 55 - 70 mm range. These lenses aren't the best either. Although Olympus gets very good reviews for it's kitlenses.

I'd advise that you go to a shop, look around and try-out some camera's. You must understand that you choose a system. Most dSLR users invest in better lenses over time and the lenses outlive the camerabody. So you'll have to stick to lenses for you brand. ( ex. you can't put Canon lenses on a Nikon)

Basically you should look for the best camera ( MP's, abilities, functions, etc.) for your budget.

Entry camera's are:

Canon 350D and 400D ( rebel XT and Rebel XTi in the US)

Nikon D40 and D40x ( but these are meant for 'normal' users, if you intend to go further in photography, you'll want a better cam like the D80 + they don't have an onboard focusmotor, so you'll have to buy lenses with this engine and you might guess that these are more expensive)

Pentax K100D Super

Olympus E410 and E510

Sony Alpha 100

Some only have 6 or 8 MP instead of the nowadays usual 10 MP, but that isn't a problem until you want to make large prints of your photo's.

Check www.dpreview.com for more reviews.

Also check out prices for lenses and accesoires, because one brand is more expensive than the other.

Basically, there aren't any bad camera's out there. You have to choose what you like.

As far as lenses go, usually 300 or  400 mm zooms are used for aviation photography. You can find cheap zoomlenses and more expensive ones. Once again, you should read reviews to decide what lense will meet your expectations and budget.

www.photozone.de

www.fredmiranda.com

Enjoy your search!

Omag

Also think about the following:

Are you into photography allready? Or are you just a beginner?

dSLR's are good quality-wise, but it can become an expensive hobby. I've currently spent about 750 Pounds on my 400D  kitlense and a Canon EF-70-300 mm lens. I'm going to buy a lense with a big aperature for low-light conditions ( Canon 85 mm f1.8 or Tamron 28 - 75 mm f2.8) + an external flash. This will set me back another 450 Pounds. And this combo still won't give me all the possibilities my bridge-camera had.

If you are on a tight budget, you might want to check out birdgecamera's, who give you dSRL-like functions and settings, without the need of buying extra lenses.

For example the Panasonic FZ8 or FZ18, give you lot's of possibilities for a fixed budget. They are great to learn about photography an will avoid you spending heaps of money on a hobby you might even not like in the end.

But if you're dedicated and have the budget, do go for a dSLR. 

I have the FZ5 and although there is some noise, you can allways correct that using a denoise-program.

My fz5 has a 432 mm equivalent zoom, which is more than adequate for aviation photography. All my shots before december 2007 are made with this camera. Plenty of examples around. 

It is taken with the FZ5, one of my first spottingtrips to EBBR.

I'll never give up my trusted FZ5. 

cheers pepper, nice to hear such reassurance from an actual user of the product!

impressive, gonna go to the shop next weekend, when my bike will be in the shop being serviced, and check the FZ18 out for myself, looks good, seen some testers shots, want to verify these and make sure it fits well. Then, once i have enough! and some time, i shall cycle(great minds think alike, i live about 45 minutes cycle from the airport, so really easy to get to!) upto BIA(bristol airport) and see if i can improve on my first shots i got there!

as mic is saying, if your really sure about this then a DSLR might be an investment, although i cant give you any advice on that.