Hey, this is qDot, maker of the SeXBox project.
Just saw that you were gonna try the hack.
First off, the PS2 and XBox mods are equally easy. Just watch out when putting the PS2 controller back together, it's a real bitch to get the back triggers lined up.
Really, the fuse depends on the wattage of the motor you're hooking the line to, and how safe you wanna be. USB can only put out 2.5W (5V@500mA), and I'm really not sure how much of that the vibration motor in the controller uses. However, most toy motors are gonna use much, much less than that. I'm working on version 2 of the mod right now (might be done next week sometime), which will include a switch so that you can use either the vibration motor or the tap lines seperately. It also uses a 3/32" jack (a lot of sex toys come with vibrator eggs that use this jack size) for connection, so you don't have bare wires sticking out of the controller, and so you don't have to risk exploding batteries by accidently applying current to one of the battery ends.