Well, it can be done......I am proof of that! It really all boils down to which airport you want to gain access to. JFK, LHR, FRA.....well you can forget about those and others like them. From what I have been told, LAX is a little more laid back, but thats not to say that you will get in though. Some of the smaller non-international airports may not be as hostile to photographers being escorted on to their ramps......it never hurts to ask.
My beloved IAH has a reputed hatred of photographers, yet I have been able to get access with the help of several friends who work for a large Houston based carrier, that utilizes IAH as a major hub. The secret is to keep a very low profile, I normally shoot from the ramp vehicle that I am riding in. I keep my equipment to a minimum normally one camera and a short to medium zoom sometimes I bring along a small Leica M4P rangefinder with a 28mm lens attached. I never take my big lenses, as they would attract too much attention and possibly get my escort into trouble. Again the key is to be invisable, walking around looking and trying to compose that shot of that Qantas 'Wuanala' will get you a** run over by a speeding tug and your escort fired (I have seen it happen to a worker on the cargo ramp @ IAH). Those unfamiliar are not aware that this is a very fast paced and dangerous enviroment.
I suggest you befreind someone who has a certain level of authority, i.e. a supervisior who on his or her day off will take you out to the ramp. But remember always do as you are told, you are there at their discression and in some cases these employees might be risking their jobs by taking you on to the ramp.
"Show me the Braniffs"