Although the instructions for upgrading the Sony Ericsson Xperia Arc (not the new S version) are pretty detailed there are some twists that are not documented:

  1. You need to upgrade to the latests official Sony Ericsson rom (currently “4.0.2.A.0.62 Generic Global World” ), you can find links in this xda developers thread, else the phone will not be able to unlock the SIM card
  2. To flash the rom you will need FlashTool (Java based app, works fine in Linux). And when it says that you have to connect the USB cable, and press a button, you don’t have to press the power button at the same time
  3. The version of CyanogenMod is not usable on the phone I installed it on, it kept crashing on the first screen, apparently the fix is to remove an app using adb

I ended up using which works fine, except vibrate.