Prop locks are a good idea; no soloution, but part of the soloution. Better cabin locks would be a good start. The key to my suzuki samurai will open a good share of the aircraft on the ramp. I know for a fact it will open a lot of commercial aircraft. That's not good. I have a key ring with about 200 keys on it that will open almost anything on the ramp, anywhere. Also not good.
I visited a FBO tonight, and saw a Learjet leaking fuel all over the place. It was in a dark part of the ramp. There had been a line change, and the new lineman hadn't seen it. As I approached the aircraft, I noted that the hellhole hatch was open, the 02 filler door was open, and the engine latches were undone. Not something a professional mechanic would do. (Turned out the crew did it, then left it like that and flew home). Possible evidence of sabotage or vandalism. I reported it to the new line shift, showed them what I'd found, and left it in their hands. Otherwise, likely nobody would have checked.
In that particular case, suppose someone had been in the process of doing something with the aircraft, and I had interrupted them? Granted, it was the fuel leak that drew my attention, but the open hatches also were a red flag. Anything unusual should be reported.
I'm still amazed that pilot certificates don't have photographs on them. There's no excuse for not having photographs on the certificates, along with additional security features. The new certificates are a step in the right direction there (except for the mechanic certificate, which should have Charles Taylor on it, instead of the Wright Brothers...

). Minor, but still something that should be addressed for security purposes.
Many will disagree, but after September 11, no aircraft were permitted in the air without a flight plan and a discreet code. Whatever one's feelings on this, clearly any aircraft that hasn't published it's intentions under such a system is in violation, and a potential risk. I don't think that's such a bad idea...any aircraft that simply flies without publishing it's intent (not really that much different than filing a flight plan) can be considered nonpartipatory, and dealt with as a threat. A couple of days after 09/11 I was arriving in Portland and noted a Cessna 185 was forced down just prior to our arrival, for that very reason. Strict, but it works.
There are many, many ways that security can be improved for little cost, without having to infringe or hamper our flying system. I'm not saying that specific flightplans, authorizations or clearances, and discreet codes are necessarily the way to go, but it's an example of a way to increase visibility and to identify nonparticipatory aircraft. It's not perfect concept, and there are trouble areas...but the point is that there are a lot of steps that can yet be taken to further advance security and protect general aviation...if not from the actual threat of terrorism or interference, then the public perception of it.
I had a habit as an instructor of watching people preflight after I had squirreled away a tennis shoe or stuffed animal in a cowl, or put car keys in places that the person should look, but probably wouldn't. Showing them the object after they had "looked" served to reinforce that it's important to look, feel, reach, shine a light, and be thorough. It's not enough to simply bend down and look under the airplane; it's important to crawl under there and examine the gear well. Usually it's bugs and critters and bird nests I find, but who is to say it won't be an IED (improvised explosive device) that's there? What's it going to hurt to look, and the hydraulic leak or seep you find there could save your life or save some dollars.
SIDA badges work well in a SIDA area, but I don't see any reason that a national badging system couldn't be effected with little effort or cost that provides a visible badge with security features, photo ID, and fingerprints on the back (or a thumb print) for immediate verification, on a national level for avaition in general.
I'm not one for giving up freedoms, but unless we take steps to protect the privileges we now enjoy, we may lose them. We can do so without greatly altering the freedoms we now enjoy, and there are a lot of ways this can be done.
The US Navy utilized a specialized team of operators years ago called Red Cell, that simulated terrorist attacks at military installations and secure facilities. They practiced planting dummy IED's, taking down facilities, taking hostages, and exposing holes in security. They were very effective, though not well received due to the clear evidence they provided that security is more often a joke than a fact. Today, the FAA has teams of personnel that visit airports employing SIDA's, who sneak onto ramps, walk into facilities, etc...to see who challenges them, and how far they get. But we can do more, and should do more, in the GA end of things, too. We could very easily have teams of individuals who help create an awareness of our vulnerabilities in the industry, and use it as a teaching tool to begin sewing up the many holes that exist. Unfortunately, the FAA has clearly painted a negative picture by striking out at people like the college student who planted the knives...it shouldn't be a private freelance effort, but an official effort, and any exposed weaknesses should be noted gratefully.
The field is wide open for improving security. It's so full of holes right now that lack of security is the watch word...anything resembling security is the hole, presently. Sort of like putting a narrow strip of fence in the middle of a cow pasture and hoping the cows will stay put. That's the existing security. We need a bigger fence that's more visible, better known to all, and we need the input of the industry to do it. Otherwise, we're all going to suffer.