This is from the sourceforge page for the libusb-win32 drivers (http://sourceforge.net/apps/trac/libusb-win32/wiki
"Vista/7 64 bit are supported from version 126.96.36.199 since a Microsoft KMCS accepted digital signature is embedded in the kernel driver libusb0.sys. libusb-win32 based device drivers can also be submitted for Microsoft WHQL testing. Several companies have successfully finished the WHQL testing with their libusb-win32 based driver package. This will allow the customers to install the driver without a warning under current 32bit/64bit Windows."
This is great news!!
I generated an .inf file for the USBTiny using the x86 inf-wizard.exe and libusb-win32 188.8.131.52. These drivers install and work properly on my Win7 x64 system.
There is still a warning about the publisher ("Windows can't verify the publisher of this driver software") but I can select "install this driver software anyway" and there are no warnings or fatal errors about digital signatures.
Finally, working x64 drivers for the USBTinyISP without any fooling around with DSEO / digital signatures! :D
wayneft, thanks for the push in the right direction that got me to figure out how to fix this. :)