I've tried this update with 1.1.00.18040 and the unit seems to freeze after connecting to USB. Seems to happen consistently.
update: here's what happened to me...
after using PRSCustomizer I was getting lockups when connecting to USB. I could still see the windows drives, but in Sony's software instead of reader it would come up as "Error" and nothing else I could do. Firmware updater wasn't seeing the device either.
To fix this, I downloaded 1.1 Firmware from
http://www.prscustomizer.com/, put it on the SD card with the rest of the files from "sdcard" folder and renamed it to "new_opt.img".
Rebooted, pressed 4, then 6 as per manual and restored the firmware to original. After that everything went back to normal, USB worked, etc.
update: seems like i'm out of luck... tried a few more times from clean 1.1.00.18040 and same result - freezes when connecting to USB.