[AT&T, Rogers HTC One X, Telstra One XL] PeterGuru:
I put together a detailed GUIDE on how to do all the following:

Change your device Cell ID (aka CID) to the generic 11111111

Re-lock the bootloader

Install the latest AT&T firmware, version 1.85

Re-unlock the bootloader

Install Clockworkmod Recover v5.8.3.1

Re-root your device

Remove most bloatware

IMPORTANT NOTE: YOUR DEVICE MUST ALREADY BE ROOTED TO USE THIS GUIDE.

If you're not rooted, and already on firmware 1.85, you're out of luck. You'll have to wait for a new root exploit before you can proceed.

Although the GUIDE applies specifically to the Rogers ONE X, it should work for most AT&T units.

If anyone notices a mistake, let me know so that I can amend the GUIDE accordingly.

It's 40 pages long, and about as noob-proof as you can get :)

LINK: HOW TO MODIFY YOUR ONE X

Peter

Addendum: Special thanks to the following members (full thanks provided within the document), without whom I could not have completed this challenge:

grankin01

scrosler

Football

kennethpenn

designgears

JSLEnterprises

niceppl

cao0215

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ADDENDUM: July 11, 2012 @ 1:40pm local time ...

Changed authorship from PeterHTC to PeterGuru. Yes, I changed my name to make it more generic. Yes, I currently own a Samsung Galaxy S3 LTE, but I still have my ONE X. When HTC comes out with their next flagship device, you can be sure I'll get one. In the meantime, I will monitor this thread daily to make sure that I address any of your concerns/questions in a prompt manner.

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ADDENDUM: June 27, 2012 @ 8:15am local time ...

Corrected a couple of minor formatting errors (thanks to xda member funkswing for making me notice the errors).

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ADDENDUM: June 12, 2012 @ 9:05pm local time ...

Added a warning on the cover page to advise that having a rooted device is mandatory BEFORE one can begin any of the installations and/or procedures within the GUIDE. (thanks to xda member Jamesyboy for making this suggestion).

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ADDENDUM: June 8, 2012 @ 7:30am local time ...

Corrected a few mistakes, and provided a bit more information in Steps 5 & 6 (thanks to xda member galfert for pointing out the errors and for providing a missing image).

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ADDENDUM: June 7, 2012 @ 8:24am local time ...

Fix and improvement of Step 6 - Install Clockworkmod Recovery (thanks to xda member addhyperactive for pointing out the error in this section).

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ADDENDUM: May 31, 2012 @ 12 noon local time ...

Further improvements to navigation.

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ADDENDUM: May 30, 2012 @ 2:56pm local time ...

Revised and re-formatted. Added a "Thank You" section on page 2.

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ADDENDUM: May 29, 2012 @ 2:55pm local time ...

Completely revamped. Added a Table of Contents with clickable links for easy navigation. Added both QUICK and MANUAL installation instructions.