Follow-up!
I had planned on getting a straight-fluted extractor over the weekend. The weekend got away from me and I never did. I did find my penetrating oil out in the garage and began saturating the bolt. Several times a day, I'd walk by and apply a bit more.

We had other distractions for the first part of this week, so I didn't need my bike. Yesterday, my wife's van came back from the shop and I returned the loaner car. I had to have my bike ready to get to work today. So, I decided to take another stab at extraction, last night. I again, tried the extractor that I picked up last week. I got it wedged pretty well into the bolt (sticking out the other side, even) and began applying more pressure. This lasted just a few minutes and then the extractor snapped. It had become a smooth cylinder with threads firmly wedged into the hollowed out bolt. Finding purchase to turn it in either direction was beyond difficult.

I decided that I just need to remove the extractor and take this up at a later date. So, I got my Dremel tool and created flat spots on either side of the cylinder, used a crescent wrench to turn the extractor bit and back it out.

While doing this, I realized how little material seemed to be left of the hollowed out bolt. For fear of damaging the frame's threads, I didn't want to use a fatter bit, but as suggested farther up, if I could get one side of the bolt to break and collapse in on itself, I could get it out. (Bill suggested using a chisel above.)

My cordless drill turns much slower than my AC drill and it has a more responsive trigger. I loaded up a bit that was smaller than the hole going through the bolt and put it all the way through. Then, I kept the drill at a slow pace and used it as a saw, applying pressure sideways. It was carving away tiny shavings of the bolt and I would stop very frequently to see how thin the bolt was, on that side.

Eventually, I cut entirely through to the frame. Then took a tiny flat head screwdriver, (like an eyeglass-screwdriver) and used a jeweler's hammer to gently tap it under the now exposed edge of the bolt. Once it was wedged in, I was able to pry up just a bit more of that edge of the bolt. It took two more passes with the tiny screwdriver to get the bolt folded in on itself. Driving a new bolt into the hole pushed the now hollowed and collapsed bolt out the other side.

It was after midnight, so I thought I'd wait until tonight to put my rack and fender back on. My wife then reminded me that it was supposed to rain this morning, so I stayed up and got everything back onto my bike. It did rain on my ride to work so I was glad to have the fender back in place.

As much as the ideas to get the broken bolt out were all helpful, I think the words of encouragement to stick with it were what really got me through this without giving up. Impatience would have been my undoing, if left to my own devices.
Thanks again, everyone.