Here is a 64-bit signed libusb driver that should work on Win7 x64 without rebooting to "allow unsigned drivers" mode:
There are also followup posts saying it worked for others too. If anybody tries this, please report back with your results. Thanks.
EDIT: I just installed it. You have to select an Atmel microprocessor device from the list when you install it. Perhaps we need to modify a .inf file before or after installation, modified to support the Kindle USB VID/PID number.