This file (3mb ish) contains the unmodified boot.img dumped from the phone, as well as boot.img.insecure, which is modified to turn off security (exact details here)
I've been testing this with the following method. Obviously if you try this it is at your own risk. You'll need a copy of "fastboot".
1) Decompress zip
2) Attach phone to USB.
3) Turn phone off.
4) Press and hold the back button, then press "power".
5) You should be in the bootloader, and it should say "FASTBOOT USB".
6) On your PC, run "fastboot boot boot.img.insecure".
The phone should boot normally. However, if you do a normal "adb shell", that shell will be running as root.
Note that this doesn't actually flash the phone; it simply uploads a kernel and tells it to boot it. On reboot, it will return to the original secured rom stored on the phone. Of course if you make changes to the filing systems as root, those will be stored to flash.