Oh frabjous day! Caloo calay! My Kindle Touch is unbricked, freed from the black screen of death! I'm so grateful to the friendly wizards on this board, especially hostar and double especially geekmaster. You have achieved amazing things. Let me put down the steps precisely, in case it might help someone:
* First, I installed hostar's windows port of yifan lu's fastboot tool, fastboot_win_alpha. That was the key. The only complicated part was the libusb-win32 installation, but I got through it by following the instructions on the sourceforge page carefully:
http://sourceforge.net/apps/trac/libusb-win32/wiki
geekmaster's screencaps should also help.
* Then I booted into fastboot mode, for the thousandth time, but this time to actually do something. In case you don't know, you hold down the power button until the light goes off, and then, without releasing, press the home button and then release the power button.
* Then I ran the following commands:
fastboot flash diags mmcblk0p2_ssh.img [from the first post in this thread]
fastboot setvar bootmode diags
fastboot reboot
That got me into diags mode, for the first time in weeks, and my heart sang!
* I started ssh, following geekmaster's instructions to "Select menu items N) U) Z) X). Wait 20 seconds for dropbear to startup"
* In linux on my mac os x, entered ssh root@192.168.15.244, with password mario. It complained that the device had a different signature or some such, so I had to delete the line containing 192.168.15.244 from known_hosts, at which point I was able to get in.
* I'd forgotten to copy mmcblk0p1.img to my /mnt/us, and I discovered that the USB mode option in diags didn't work after starting ssh, but on choosing Reboot (D, R from the main menu) it did work again. Then I went back in with ssh.
* dd if=/mnt/us/mmcblk0p1.img of=/dev/mmcblk0p1 bs=4
which took 137.6 seconds.
* Now to reboot into main. Disable Diagnostics didn't work, with the message "device_info.xml not found, check device settings", but no problem with my new fastboot powers, I just chose "Install fastboot bundle" and then ran fastboot setvar bootmode main, fastboot reboot.
* The normal, healthy Kindle reset screen, and the same screens I saw when I first bought it. I wonder if this was affected by me running fastboot setvar bootmode factory at one point. I had to unplug and plug the usb back in to get the normal directory to mount, but when I did unlike when I exported it before, it was populated with the expected folders.
And then the home screen came up!! After more than a month, and my girlfriend giving me a $140 Amazon gift certificate for my birthday (so little faith!) it's resurrected. Have I learned my lesson? Not on your life! As long as I don't step on the thing, I now feel like my Kindle is INVINCIBLE! I want my awesome Mac Plus-themed screen savers, and I want my keystrokes not to be logged. Now back to where it all began, with Yifan Lu's mp3 hack...