I'll tell you honest, I've had a drink and a time down at the bar prior to posting, but I'm pretty straight, so bear with me.
RIGHT! What we have here is an idea I got from an aside in one of Unbreakable's posts (Unbreakable, good guy!) where he noticed his plug was filed flat on the top of one of his locks. He was wondering why and happened to mention, off-hand, something to the effect of "I know this isn't to make one key lock it and not unlock it, as it's not filed properly" or something to this effect.
Soooo...I searched around a bit, but couldn't find anything immediately relevant on the subject.
Not to worry! Cause I pulled out the plug and thought "OK - file it, and make it work in one direction, but not the other with this here key, yah..."
I figured it out! It didn't take all that long to understand the concept, though I doubted myself a few times. Anyhow, now, for your viewing and reading pleasure. My lock that opens with the green key and closes with the yellow key! DUN DUN DUN!
Basically, whatchagottadoisthis:

andabittathistoo:

Basically! You file down two of the chambers in the plug in opposite directions. I chose the second and fourth for my operation. I used a couple of files, one was flat with a round bit, (one side rounded, one side flat, rather) to get me started, and one was square, to clear out the bits in the corners that were keeping it from operating cleanly.
Right-O - so to continue the explaination 2 MORE PICTURES! GIVE IT UP!


This would be the green team, showing it's stripes. So, as you can see, on one of the filed down chambers, the bottom pin only rises up to the break between the top pin and the filed down part of the plug. But, on the other one, it rises up the 100% honest, FOR REAL, sheer line. Now, all you swank cats and kittens can figure out what this means, TO BE SURE.
Basically: The key can turn in the direction the plug had been filed down (the plug with the pin rising to the file marks, rather than the sheer line) but it CANNOT turn in the opposite direction, as the pin that rises to the sheer will happily keep moving in either direction, but the pin that rises to the filed portion will find itself all bound up when trying to turn against the filed direction.
I know, I know, you see it, I should get on with the sweet pics:


Now, I hope this brings it home for everyone. The yellow key is cut in the opposite manner, with the pin that previously reached the sheer, now reaching only to the file line, and the filed line pin now reaching all the way up to the sheer.
Hot Dang, I think he's got it.


The keys for your viewing pleasure. You can see a bit of the green behind the yellow (where the cuts change) and a bit of the yellow behind the green (ditto, baby).
Now,

both keys for comparison sake (these are hand filed, thus the aluminum blanks, super swank for hand filing experiments.)
And!
http://www.youtube.com/watch?v=Db80AqoV-FY
That's the link to the divx formatted video of it in action. You can guess what happens.
[EDIT: Updated video from 2010:]
http://www.youtube.com/watch?v=Db80AqoV-FY
so, So, SO! Why bother?
Well, as I was beginning this project (scrapped one plug when I junked it up) I mentioned to my boss at work that I wasn't quite sure what the practacle use would be.
He responded,
"Well, Schuyler, this would have been great for us, as, when we finally gave you some extra responsibilities, we were comfortable with you locking up, but VERY hesitant to give you a key that would let you into the place any time you wanted..."

anyway, this was a very fun project. I just got another order of 125+ locks in, so hopefully I'll be having some other fun plug mods in the relatively near future.
Also, if anyone's in NC - I need a place to stay tomorrow night. I'll be arriving in Wilmington at like 10:30, if you're sympathetic: show up and act like ya know me. I'll be in a suit and a cowboy hat.
HECK YES! Post it, LP101, POST IT!