1. must be s-off (see link)
2. must install this hboot 1.09 from this guide
3. ICS RUU file (see this link). In my case, I downloaded the earliest version for the Telstra RUU so I'm sure it's ICS.
4. working adb/fastboot files (this one worked for me)
5. twrp recovery (link). you can use the recent cwm I suppose but I haven't tested that.
1. open a command prompt/terminal
2. follow the steps in requirement 2
3.. boot into android
4. adb shell
6. dd if=/dev/block/mmcblk0p23 of=/sdcard/p23
7. get out of adb shell (run exit twice)
8. adb pull /sdcard/p23
9. make a copy of p23
10. open p23 in a hex editor
11. look for a sequence that that start with 3.17 (complete format is 3.17.xxx.x. x are numbers)
12. replace 3.17.xxx.x with 1.00.000.0 and save the changes to a new file. In my case I saved it as p23mod.
13. adb push /sdcard/p23mod
14. adb shell
16. dd if=/sdcard/p23mod of=/dev/block/mmcblk0p23
17. get out of adb shell
18. execute RUU as let it run its course. Took about 10 minutes in my case.
19. install your chosen recovery (boot into fastboot and then fastboot flash recovery my-recovery-here.img)
20. repeat step2 (only if you want to remove the red text in the startup screen)
1. These are the exact steps I followed to get it working on my phone.
2. steps 1 to 17 might be optional but the RUU install checks for previous version and I'm not sure if it'll go through if it finds a higher version is installed already. You can obviously experiment on your own.
3. A nice surprise after reinstalling stock ROM via RUU is that the "Tampered" text was removed and never appeared again even after I flash TWRP.