I'm a little bonkers about security as well (password length in the dozens!) as my mac is very much a public resource in a dorm room that regularly has 8-12 people in it. I don't want anyone fiddling with it when I'm not there, but you have to remember that there are some things you CANNOT protect against. If they have local access and they want to screw you bad enough, they can do it. Removing the hard drive and whacking it with a hammer are the biggest ones that come to mind, and there is very little you can do save locking your computer in a safe. Open firmware password? disabling core dumps and protecting your memory? You name it, a motivated cracker can bypass it. Like you, my security is very much for my own benefit.
That said, my newest trick is a network of webcams that I make motion sensitive when I leave. I got the demo of SecuritySpy and did some applescripting to pop up a password dialog, and Automator, to make all this available from a contextual menu. You could put a folder action in the hidden /Volumes folder to lock the screen when it doesn't detect the drive's name, but then all someone needs to know is the name of it and they can pop in anything that'll bypass it. You could script something that looks for the drive's name, then accesses a file with a random string of characters or something, locking the screen if this isn't found. I've never actually done this, so I can't actually assure you that it'll work. But fiddle around; applescript is fun! 🙂
