First I tried to do testpoint (with S1TOOL) on my Arc S and I get: NOT RECOGNIZED SIMLOCK CERTIFICATE.
After I followed this tutorial to convert .img to .ftf and try to flash with flashtool:
I converted the lastert official LegacyXperia CM11 nightly to .ftf and tried to flash with flashtool.
(If you have an error like C:\Flashtool\devicesyourdevice\loader_unlocked.sin (System can not find the file specified) in flashtool, just grab the file loader_unlocked.sin from R800 folder to your device folder thread link: http://forum.xda-developers.com/show....php?t=2207610 thx to Abhinav2)
Later I was thinking why it worked?
R: I just compared latest R800 ftf's loader.sin with latest LT18i ftf's loader.sin using WinMerge and it said that the files are identical!
So if you flash the R800 loader_unlocked.sin (in Flashtool's device folder you can find it) on you locked bootloader phone it will unlock your bootloader.
PS: Even if you not have fastboot access you can follow the tutorial to convert .img to .ftf and flash it. It will works!
Sorry for my bad english.