n900<3: You must be running firmware version 3.41; this is the only one that the exploit will work for.

(It's a heap overflow exploit, in layman's terms what it does it break out of the section of code where the USB handling code is, and get into an area xxx bytes away to access some other part of the code. As each firmware version is different, the xxx number of bytes is only correct for the version it was written on, 3.41)

In other news, Sony has just released 3.42 which closes the exploit. Do NOT use your PS3 to update if you want to run this exploit. Find a copy of the 3.41 firmware online, copy it to USB key, and tell your PS3 to use a USB device for the firmware update as opposed to using the internet.