I'm still working on getting fastboot talking to my bricked (black screen) KT. As geekmaster suggested, I'm now working with VirtualBox rather than VMware player, and it seems promising, in terms of not kicking the Kindle out of fastboot when I switch over. I might be close! But there are some things going on with the way virtualbox handles USB devices that I don't understand.
I'm running Ubuntu in virtualbox (running on Windows XP), and when I boot the Kindle into Fastboot mode it shows up in the list that comes up when I right click the USB icon in the lower right (Amazon Kindle [0001], Vendor ID 1949, Product Id: D0D0). It is listed as "Available". I have added this device to the list of USB Device filters, but left it unchecked for now. When I select the Kindle from that list, I get the tone signifying it has disconnected from XP. It is now listed as "Captured" in the virtualbox menu, and hovering over the USB icon it says "No devices attached." Nothing extra listed in lsusb, and fastboot tool says waiting for device.
Nothing seems to change when I check amazon kindle in the Settings window, except that sometimes when I go through the procedure it gets loaded as Unknown device:1949
0D0 with the status "Held".
So I believe that my Kindle is probably staying in Fastboot mode, but virtualbox isn't set up to talk to it. Two facts that might be pertinent: First, in the virtualbox USB Settings, both Enable USB Controller and Enable USB 2.0 EHCI Controller are greyed out, with Enable USB 2.0 Controller unchecked. Second, when I first booted to fastboot with Ubuntu up, windows asked about a new device called something like "VirtualBox device", and I downloaded and installed the driver it suggested. I feel like before that happened, I *may* have successfully had Amazon Kindle connected to the virtualbox, but then it went away when that was installed. However I'm not sure of that - the sequence of events was very fast.