Go to: http://unlockbootloader.sonymobile.com/
and read everything!
At the bottom click 'Start Unlocking the Bootloader'
First, you should confirm that it is possible to unlock the boot loader of your phone by checking the service menu. In your phone, open the phone dial pad and enter *#*#7378423#*#* to access the service menu.
In your phone, tap Service info > Configuration > Rooting Status. If Bootloader unlock allowed says Yes, then you can continue with the next step. If it says No, or if the status is missing, your device cannot be unlocked.
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
From here- http://forum.xda-developers.com/show....php?t=1753261
Is the same for any ION. Some can be unlocked, some can not be unlocked.