I'm posting the solution I found for anyone else who's having the same problem that I did.
I bought Minecraft during the Beta, back in October 2011. I've been steadily upgrading since then.
I've had a very difficult time finding a Nether Fortress single player survival mode. I deleted the DIM-1 folder several times, and then spent hours exploring the Nether to no avail. Eventually I got some of the mapping tools to look around and found out that there was indeed no Nether Fortress.
1) I put all of the equipment on my character into chests and made sure that I wasn't in the Nether.
2) I started a new game (in version 1.2.5), saved, and quit.
3) I went into the save directory for my new game and copied the 'level.dat' file. (Windows: type %appdata% in the run box).
4) I went into the save directory for my old game, and backed up the old 'level.dat' file.
5) I copied in the new 'level.dat' file over my old one.
6) I deleted the DIM-1 subfolder in my old game.
7) I restarted minecraft.
8) I used "rename" to fix the name of my old save game.
9) I started the old game.
I started a distance from base, naked*, and way up in a tree. I got back to my Nether portal to rebuild DIM-1, When I checked the new Nether with my mapping utility, I found that I now had a Nether Fortress. Now I can get back to the game.