[Sony Xperia S, Acro S, Ion] Spectre51:
Taken from [Tutorial] Xperia S/P/U/Sola Bootloader Unlocking & Relocking and modified slightly for the ion. I take no credit for this tutorial.

Quick tutorial for unlocking & relocking the bootloader of the Xperia ion.

This is the official way to unlock bootloader

Warning!

This procedure will wipe the /data partition of your phone, so you will loose all ur stored data, settings. SDCARD will NOT BE WIPED!!!

DRM keys WILL be lost forever

Why should I unlock the bootloader?

You can flash custom ROMs/Kernels.

You don't need to worry if the new firmware can be rooted or not as with unlocked bootloader you can always ROOT your device.

SE/Sony gives sneak-peaks/preview builds of latest OS which requires unlocked bootloaders, as they did with Xperia 2011 line (they had ICS alpha/beta), so who knows may be SE/Sony may give early builds of ICS (JellyBean???)

Most of this is taken from lollylost100's his tutorial from ARC thread (all credits for that concise tutorial go to him, i have elaborated on a few things and added device specific changes)

[ Before we begin unlocking bootloader ]

Get the IMEI code for your device: type *#06# in phone dialer to find it out, note only the digits (no dashes)

Download the fastboot package with usb drivers (alternate drivers if the ones in package dont work)

Make sure u have a windows PC with USB cable to connect the phone

Power down your phone and keeping the Volume Up button pressed connect the USB cable to PC this will trigger FASTBOOT mode and windows will start driver installation, extract the above package and point the driver install to that location (usb_driver) folder

Once drivers are installed just disconnect cable and power on device

[ how to unlock ] (official way)

Go to: http://unlockbootloader.sonymobile.com/ and read everything!

At the bottom click 'Start Unlocking the Bootloader'

After you have verified that you can unlock bootloader, scroll to the bottom of the page and click "Continue"

You will get "Are you sure prompt" accept it by clicking "yes i am sure" (i assume that you have read an understood it)

Then accept the "legal terms"

Finally you will get a "Create Unlock request" page, enter your IMEI (sometimes the last digit needs to be removed for this to work), name and email address

You should now receive your unique unlock key, its displayed on screen, SAVE THIS KEY SAFELY

Extract fastboot_with_Android_USB_file.rar and open fastboot folder

Hold SHIFT and right click on the background of the folder

Select 'Open command window here'

Connect your phone in fastboot mode (turn phone off, hold Volume UP button and plug in USB Cable) [make sure drivers are installed as explained above)

In the cmd windows type in fastboot.exe -i 0x0fce getvar version

If a value is returned (it will return 0.5), it means the phone is correctly connected

Type in fastboot.exe -i 0x0fce oem unlock 0xKEY and replace KEY with the key which you received earlier.

Your device should now be bootloader unlocked

If this tutorial is too much for you then itskapil wrote a step-by-step guide (and recorded a video also):

-Instructions: http://www.theandroidsoul.com/how-to...-play-neo-pro/

-Video: http://www.youtube.com/watch?v=LRH3pC7MhEA

(the above videos and some steps are for Xperia 2011 devices, but most the steps are same for Xperia ion)

Info on Relocking bootloader is in next post