I've been wondering the same thing. For three days I haven't been able to get it to work on two different machines. I tried renaming a custom 2.1 fw, by removing the _Custom from the name, then I put it in the folder that the official fw goes to, then restoring. (shift + restore not bringing up the browse window). This only resulted in errors 1600 in DFU mode and 6 in recovery mode. Currently I'm stuck with a standard issue 2.1 fw. 😕