I cycled into town today and had some copies made. It's funny, but my house key looks identical to the graphic Benson posted. When I got home, I put the key in a vise, taking care not to crush it. Then, I used a hacksaw to carefully cut off the head of the key, and some sandpaper to remove the sharp edges. I'll post some pictures of the before and after in a bit.
I went to my front door and tried it, and it works! The only downside is that without as much metal to grab onto, it is a bit harder to turn the key in the lock. But in all honesty, the reassurance of being able to get in, the DIY factor/the fun involved, and the cost ($1.71 for the copy) more than make up for that. A small hole at the top will allow me to attach a little split ring.
Now, I just need some sort of band, durable elastic/latex to slip the key around my ankle when walking/hiking/being outside of the house without my bike/car. When I was in camp, we had these little elastic bands to identify what grade of swimmer we were. They were thin, did not pinch, and were pretty tough. I am not sure where to find something comparable. I would rather have a stretchy band than something else because they are:
1) cheap
2) form-fitting