[Xperia Arc Q&A, Help & Troubleshooting] mor87:
To unlock bootloader I did this:

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 dialler 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.

On your computer, click Continue at the bottom of this page to start the unlock boot loader process.

Read through the information in the pop up window and click Yes, I’m sure, if you are still confident you want to unlock the boot loader.

Read through and accept the legal terms. Click I Accept.

Enter your name, email address and the first 14 digits (remove the last digit) of your phone’s IMEI number. Click Submit. You can view the IMEI number by entering *#06# on your phone.

The unlock boot loader key is displayed. Write it down and save it for later use.

Download and install the latest Android SDK from developer.android.com.

Download and unzip this file. This is the standard android_winusb.inf-file, with a few lines of code added to enable Fastboot to support Sony and Sony Ericsson devices. Make sure you are running Google USB Driver package revision 4 or higher in your Android SDK. You can see current Google USB Driver package revision in the SDK Manager.

Place the unzipped file in the usb_driver folder, located in the Android SDK > extras > google folder on your computer. Accept to replace the old android_winusb.inf-file with the new file. If you can't find the usb_driver folder, please install the Google USB Driver Packager using the Android SDK manager.

Turn off your phone.

On your phone, push and hold the unlock button* at the same time as you connect the phone to the computer with a USB-cable. The unlock button may differ between different phone models. See the note in the bottom of this page for more information on what button use on your phone.

On your computer, you will be asked for drivers. Point to the usb_driver folder where you placed the android_winusb.inf-file, and select the Android Boot loader Interface-file.

Open a command window and go to the tools folder within the Android SDK folder.

Enter fastboot.exe -i 0x0fce getvar version, and verify that you get a response back (for example "0.3"). This means your phone is connected properly.

Warning! If you perform this step, you will unlock the boot loader. This may void your warranty and/or any warranty from your operator. If you still want to unlock the boot loader of your phone, enter fastboot.exe -i 0x0fce oem unlock 0xKEY, where KEY is the unlock boot loader key you got in the beginning of this instruction. The boot loader should now be successfully unlocked.

per cambiare kernel ho fatto questo :

Download the latest version of the rom.

Copy the zip file downloaded to the phone's SD card.

Open the downloaded zip file from step 1 on your computer and extract the boot.img file from inside the zip to the tools folder in your ADB on your computer.

Boot into fastboot mode on your phone. To do this, turn off the phone. Then, while holding the Menu button, connect the USB cable to the phone.

Now flash the boot.img (this is the kernel that are required to update the ROM) with fastboot by typing the following command:

fastboot flash boot boot.img

* or if the previous command does not work, then enter the following command:

fastboot-i 0x0fce flash boot boot.img

After the flash is complete, disconnect the phone from the computer.

Now, start the recovery ClockworkMod on your phone. To do this, turn on the phone, then wait for the blue LED to illuminate. Once you see the blue LED lit, enter the return key on the phone to enter CWM. In recovery, use the volume keys to move up and down and the home button to select the options.

I also followed the video you posted ..

but nothing to do ..

could you advise me :( ?