Here's a better (yet more expensive) idea. This idea requires at least two people, and probably at least $50,000, but I think it'd have a better chance of working.
You start by building a single-seat aircraft. You can base it on one of the commercially available kitplanes out there. You build it with a quiet engine, paint it light gray or blue - something that blends into the sky easily, and generally try to make it inconspicuous, but without making it look like you're trying to camoflague it. You have to make one change to the design of the plane though - you add a bomb bay to it, big enough to drop something about 3 feet wide and 3 or 4 feet long.
The object that you're dropping is essentially a radio controlled glider. It's designed to go a lot faster with a lot lower glide ratio than most gliders. There are a couple cool things about the glider: it's silent, you can make the wings (which I'm guessing would be fairly stubby) transparent - it'd be difficult to see.
Say you design this glider to maintain 100mph or so, and a glide ratio of about 1:1. A typical R/C glider should have at least a 10:1 glide ratio, meaning that for every foot of altitude it loses, it covers 10 feet of distance. Our 1:1 glide ratio glider basically falls at a 45 degree angle. This means that if you drop it at 10,000 feet, you only have to be within two miles (horizontally) of your target. I lived near a small airport for a long time, and a small airplane like that isn't particularly noticeable from two miles away.
Anyway, your glider weighs about 25 pounds or so, most of which is explosives rigged to detonate when the thing experiences a massive shock (or when the pilot presses the "detonate" button). Other than that, it's got a battery, a radio, and a camera in the nose. Person 1 flies the plane, and person 2 watches the video feed from the camera in the nose of the glider, using that to control it. Now, this glider is about 3 feet long, and moving at about 100mph, and it's silent. By the time anyone saw the thing, assuming they were looking right at it as it came into view, They probably wouldn't have more than 5 seconds before it detonated. Even better, it's coming in from a 45 degree angle, making it well above the typical field of view for someone that's primarily concerned with someone attacking from the ground.
You've basically built a miniature smart bomb, and I think that as long as the guy piloting the bomb was able to aim it correctly, it has a pretty good chance of success.
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