[Palm Treo 750 ROM Development] dubLeE:
With this method I have successfully unbranded, unlocked, and updated 2 Treo 750s that were originally AT&T/Cingular branded.

I spent hours running through multiple threads reading and searching to finally find a solution that worked flawlessly. These steps have been compiled from multiple threads:

http://xdaforums.com/showthread.php?t=428048

http://xdaforums.com/showthread.php?t=412999

http://xdaforums.com/showthread.php?t=334100

Big thanks to Olipro, konker, and jcrompton!!!

Download the following:

Magic Unlocker from http://xdaforums.com/showpost.php?p=2468221&postcount=2

CheetahUnlocker from http://xdaforums.com/showpost.php?p=1556568&postcount=1

1_CHEE_MountExtROM.cab and 2_Chee_Extrom_rename.cab from http://xdaforums.com/showpost.php?p=1424566&postcount=374

Treo750_OS5_2_19199WWE_STD.rar from http://omnifusionhosting.net/treo750/

(I couldn't find the original posts for Treo750_OS5_2_19199WWE_STD.rar so I uploaded to my own site for now)

1. Download, extract, and run MAGIC_UNLOCKER on your phone.

2. Select both options in MAGIC_UNLOCKER and click unlock. This will unlock your phone.

3. Format your SD card to FAT32.

4. Extract Treo750_OS5_2_19199WWE_STD.rar and copy/paste CHEEIMG.nbh to the formatted SD card.

5. Transfer 1_CHEE_MountExtROM.cab and 2_Chee_Extrom_rename.cab to your Treo 750.

5. Run/Install 1_CHEE_MountExtROM.cab and then soft reset the phone.

7. Run/Install 2_Chee_Extrom_rename.cab and then soft reset the phone.

8. Click and run Start>Programs>Chee_Extrom_Unlocker. Run it about 3 or 4 time and you should some type of error messages. Just OK or X them and soft reset the phone again.

9. Open File Explorer and click the drop down arrow, in which you will now see ExtendROM. Open it and delete the 2 files in the partition folder.

10. Download, extract, and run CheetahUnlocker on your computer. For this part make sure you have an ActiveSync (XP) or WMDC (Vista) connection to your phone. Click Flash SPL and go through the steps for CheetahUnlocker to Flash SPL. Don't worry about any of the other items in the menu.

11. After going through the steps in CheetahUnlocker and the phone, the phone will enter into tri-color bootloader screen momentarily followed by Loading screen...

12. Next, you'll see a light blue screen with the heading CHEEIMG.nhb followed by a few other entries below it. At this point, it will tell you press the power button. Go ahead and press the End Call/Power button. This will begin the ROM flashing. The ROM flashing takes about 5 to 15 minutes.

13. When the flashing is completed, the bottom of the screen will say Update successful and the entries above with the OK seen at the end. Now reset the Treo 750 with the stylus and immediately (during the Palm Logo) perform a Hard Reset. This is accomplished by pressing and holding the End Call/Power button and simultaneously resetting the Treo 750.

14. You will now be prompted to Erase All Data. Press the Up key of the D-Pad to continue and erase the data. This will format the phone.

11. Upon completion, the Treo 750 will continue to boot-up and go through the initial start-up steps and wizard. Once completed, your phone will now be unlocked, unbranded, and updated to Windows Mobile 6.1.

I made this thread to help people unbrand, unlock, and update their Treo 750s without having to waste hours looking for solutions and how-to something. Basically, made it easier for the noobies and everyone else to find everything in one thread.