I have an Xperia Arc S (LT18i). How do I install Cyanogen Mod on it?
Is that for the CM9 RC1 that was released yesterday? :D
There's two things you need to do:
As for the way things are, there's no true CWM recovery for the Sony due to the inherent nature of how Sony does it (Its different!) - when you unlock the bootloader, you're also unlocking recovery BUT, the boot and recovery are residing on the one partition so do not try flash CWM into it otherwise BAM, goodbye!
There is an app on the market, for a CWM which is not a real CWM (it dumps its binaries into the /system directory) which by the way, must remind you need root for this!