The greatest CM9 is now official for Arc S users and finally, fastest, prettiest most powerful, anyway the kernel that comes with it doesn't work but the FXP126 one works with it anyway, its the same on the package but you have to download it seperatly.
Get the rom here - Cyanogenmod 9 RC1 Official
and get the kernel here FXP126
Get ICS gapps here you MUST flash this too if you want the google market, gmail, sync etc..
Any baseband should work but for me 70 was the best. you should try 72 also if you like but I don't have it so I am using 70.
And here how it looks like: