Thanks very much for this rom JJ, my Arc is finally USABLE again! Very fast, stable and reliable so far.
Just a couple of minor points, not bugs but maybe things you didn't notice. I have a locked bootloader, so followed the instructions to flash the official 4.0.4, then wiped data, before using recovery to install your rom. Several apps (mostly Sony ones I think) force close when you start them, including: Track ID, Facebook inside Xperia (inc. the friend's pictures apps, etc), 3D Camera/Gallery, Xperia Gallery, Music Likes and Crash Monitor. Fortunately none of those are particularly important and can be easily removed with SystemCleanup.
Another minor issue is that the phone now identifies itself as an LT18i instead of LT15i. I'm not sure if it's this, ICS or the use of a custom ROM, but I have noticed that one or two apps from the Google Play store (notably the NatWest app and BackStab) now claim to be incompatible with the phone.
Just FYI, Panorama beta (found here on xda a while back) for the X10 works fine as a replacement for the non-functioning 3D Camera app (I also tried the 3D Camera attached to the 2nd post here, but it also fails with 'camera not available'). I also found a few of the Xperia S apps from here
work well on this rom: CosmicFlow (live wallpaper), SemcAlbum (album, sorted by date), SemcNotes (a notes app) and SemcPhotoEditor (for use with album).
I just noticed a slightly more serious bug - in the CM9 Music app, if I select 'sound effects' in settings, Music force closes. The same happens if I try and select the graphic equaliser from the now playing screen. Replacing the CM9 Music app with the Play Music app using the zip from the 2nd post fixes this.
GT-i9505 XXUAMDC (4.2.2)
LT15i 4.1.B.0.587 (4.0.4)