[XPERIA X8 Android Development] DerEineDa:
All credits go to pulpoff2 for this! This is not by me, I am merely the messenger. Only thing I did was to ask pulpoff for the files and a tutorial :) With his permission I will share it with you.

This will provide you with a fresh and unmodified 2.1-system, exactly like you would do the official update - only exception is that it is rooted. So all the apps and games of the official firmware are available and nothing else. You can remove the crap (like the games or timescape for example) by yourself after doing the steps below.

UPDATE:

There is a newer ROM by propc you can use instead of this one. Look here:

http://xdaforums.com/showthread.php?t=888427

-- NEW --

propc kindly provides us with a shellscript to automate the steps below. See here for the script and a short manual:

http://xdaforums.com/showpost.php?p=10067921&postcount=120

If the script doesn't work for you or you prefer to do things the manual way, you may use the steps below.

Download these files:

x8-2.1rooted.img.yaffs.rar

and

ISO-8859-1__unyaffs-arm.rar

Also download the 2.1-firmware from here or here. Unpack it on your computer and delete the "system.sin".

Unpack "x8rooted.img", "busybox" and "unyaffs" to SDcard.

You need 1.6 with root on your phone. By the way, all the data on your phone will be lost after doing the following steps, so making a backup is advised.

Use "adb shell" to connect to your phone and become su:

su

Copy "busybox" and "unyaffs" to /data/local/tmp. (Pulpoff says you should copy to /sbin, but this requires remounting with busybox. I copied to /sbin, but I think /data/local/tmp should work just fine)

cd /data/local/tmp
cat /sdcard/busybox > busybox
cat /sdcard/unyaffs > unyaffs

chmod the files to make them executable:

chmod 755 busybox
chmod 755 unyaffs

EDIT: If you want to copy the files to /sbin (like pulpoff2 advises), you can do this OPTIONAL step:

EDIT2: It was confirmed that this is indeed not necessary!

Remount the root directory to make /sbin writable. Then copy the files over and change the working directory to /sbin:

[strike]
./busybox mount -o remount,rw /
./busybox cp -prf busybox /sbin/
./busybox cp -prf unyaffs /sbin/
cd /sbin[/strike]

Remount /system as writable:

mount -o remount,rw /dev/block/mtdblock0 /system

Switch to busybox shell:

./busybox sh

Remove /system (yes, really!). It's normal to get errors because of the "lost+found"-folders. Ignore the errors.

./busybox rm -r /system

Unpack pre-rooted image to system. It should output "end of image" if it finishes correctly.

./unyaffs /sdcard/x8rooted.img /system

Your phone won't boot anymore now and it will crash as soon as you try to use it - that's normal! Go to next step.

Just remove the battery and the usb-cable from the phone, then re-attach the battery. Flash 2.1-kernel (without system.sin !!) using the flasher-tool by Bin4ry. If the flasher gives you an error because of a missing "loader.sin" you can take this file from the 1.6-firmware. Refer to Bin4ry's thread to learn how to use his flasher and how to enter flash-mode on your phone.

The phone should now boot, although the first boot can take a little while. If you get into a boot-loop or you just see the white sony ericsson logo (not the green one after the animation) for several minutes, it probably didn't work and you have to start over again by flashing 1.6.

This was written as I remember how I did it by myself. It worked for me at first try! If it works for you, please click the thanks-button. It will make me feel better :)

After you have your nice rooted 2.1 you probably want to enable JIT for a nice performance boost.