Sony Ericsson has just stepped in in a right direction in customizing Android ROM. A web site is now available for unlocking bootloader specifically for XPERIA devices. That means unlocking bootloader on your XPERIA ARC and XPERIA PLAY is official to use custom ROMs.
A guide for this is now available here and NOTE not all phones support this solution including some sim-locked devices. Your handset which you are using is compatible or not can be found from this guide. This just requires your name, email address and the first 14 digits of the handset’s IMEI number for you to get started.
Well ‘Sony Ericsson recommend’s you not to unlock the boot loader’ is just an official warning from them to excuse any software failures. A legal Term box informing you about warranting voiding and what not is also available in there. You will get the needed code after all this complication.