OK - so why should there be anything illegal about it?
As long as you comply with instructions regarding not using electronic equipment when asked not to do, i.e. landing and take-off (I assume you are using digital? Don't ask for trouble using huge telephoto lenses etc on the plane!)
Also, depending on the airport being used - is it a shared military base? Photographs in those circumstances may be unwelcome - but, above all, think about it - are you taking photographs in a situation that could, obviously be a cause of concern? If not, don't worry - enjoy yourself!
Edit by BOAC: I've taken out a couple of commercial links which do not have much to do with the question!