[Sony Xperia T, TL, TX, V] sclera:
This is a beginner's guide to reduce AT&T branding on a Sony Xperia TL (model LT30at, build 9.1.F.1.120).

This phone has 2 GB phone memory (apps first go there), 11 GB internal storage (you can move apps there but not many of the branded apps), and you can put an SD card in (mine is about 32 GB for music/video). Why they split out 2GB versus 11GB forcing you to move apps that you hope the developer marked as moveable is a mystery, since it's all built-in memory.

Without rooting, this is the normal way to disable even system apps you can't uninstall or move within Android:

Settings | Apps | swipe right to All

Pick the app to disable

Force Stop it, Clear data, Uninstall any updates

After uninstalling updates, you can (often) hit the "Disable" button

If you want to go further, you'll need to root. I was able to root by:

From Windows Install Sony PC Companion

Plug phone into usb cable attached to PC

Settings | Developer options (slider on) | USB debugging (turn on)

Windows then downloads ADB drivers for you automatically

Settings | Security | Unknown sources (turn on)

Root using this: forum post

Find it at "Mirror 1 Uploaded.to" there

Unzip and run the RunMe.bat

Select option: 0) Xperia Root by cubeundcube

Afterward, Google Play store gave me "error retrieving information from server [RPC:S-3]", fixed by:

Settings | Accounts | Google | select account | triple-dot-menu select | Remove account

Settings | Accounts | Add account | Google

Give it time to resync, you can see it at

Settings | Accounts | Google | select account | (watch for sync to finish)

I didn't try a custom build, but note that 4.3 Jelly Bean build 9.2.A.1.199 (note, this is not 4.1 Jellly Bean, but 4.3 Jelly Bean) has arrived 2014-03-10 and is being rolled out says Sony's site. So I'll wait for that.

I needed a file manager which runs as root, but it's nice to have the Superuser app to manage which app is root:

I installed the Superuser app from ClockworkMod to manage who gets root and when.

Then I installed the File Manager (Explorer) app from Rhythm Software to move things the apps out of the system area.

You'll need to do settings | Root Explorer, and settings | Mount System Folders.

Then I erased the startup AT&T logo and sound by simple removal.

With file manager running as root and system folders mounted as read/write

Navigate to /system/media

Delete the file bootanimation.zip

Now on startup you get a simple default very small "Android" logo, and no more AT&T sound

I tried putting an alternate bootanimation.zip but I didn't sequence the images inside right and that almost bricked my phone when I ended up with a blank screen on reboot, and luckily had 1/3 of a screen in landscape orientation only, which I was barely able to navigate back to my replacement bootanimation.zip and delete it. That's how I discovered Android has its own built in animation, although minimal one. Don't worry, simply removing the bootanimation.zip is fine.

Then I first disabled many things AT&T and Facebook related using normal Android mechanism listed on top. But if you want to go further, to git rid of what I disabled, maybe to free up space from the scant (2GB) phone memory, with File manager (running as root and system folders mounted as read/write), navigate up and up then down into /system/app. There I removed all these safely. Hit multi, mark them and move to your sdcard:

AttAddressBook.apk

EventStreamPluginFacebook.apk

FBCalendarSync.apk

FBIAppShare.apk

FBMediaDiscovery.apk

FBMusicLike.apk

FBSetupwizard.apk

PostViewer.apk

SEMCFacebookProxy.apk

SemcTvLauncher.apk

SmartSearch.apk

SocialEngineFacebookPlugin.apk

SocialEngineFacebookShareFrwkPlugin.apk

Talk.apk

TimescapeSpline.apk

Yellowpages.apk

bn.ereader.apk

cityid.apk

com.att.android.attsmartwifi.apk

com.att.android.digitallocker.apk

com.att.android.mobile.attmessages.apk

com.att.featuredapps.handset.apk

com.att.mywireless.apk

com.drivemode.apk

com.mobitv.client.tv.apk

com.mtag.att.codescanner.apk

com.telenav.app.android.cingular.apk

com.yellowpages.android.ypmobile.apk

facebook.apk

ready2go.apk

timescaperssplugin.apk

After removing immediately restart since you did surgery. Much bloatware and branding is gone, there's more space, and no branding image or sound when it boots.

Note that the cached compiled versions of the apps don't get deleted like they are supposed to, so you have to intelligently delete various .dex files manually:

Run File Manager (as root and system folders mounted as read/write)

Navigate to /data/dalvik-cache and remove things the correspond to your deleted .apk files. The names are similar mostly.

Note the system will regenerate a .dex (compiled cached version of your app) automatically so don't worry about overdeleting.