Xperia ArcS how to install cyanogen 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:


Remove the simlock first! (Flashing a ROM with simlock on could spell disaster) 
Unlock the bootloader to enable you to run custom roms.


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!

You will need se1tool (IIRC) to flash the latest ROM, you could also check out the FreeXperia project for information on this, FreeXperia is by jerpelea, which is now what you're seeing, as CM9.