Just came across this little video
highlighting the collector's edition. Not really new information, but there's a blink-and-you'll-miss-it glimpse at some concept art for Jack's new look. I've heard rumours of who/what the secret squad mate might be that I shan't repeat since it might be spoilerish.
Personally I'm not holding my breath for anything fantastic. I mean just look at Bioware's track record for release day DLC companions; Zaeed, Shale (?) and that boring geezer in DAII. Admittedly Shale was interesting as was Kasumi (though she was a later addition...kinda) but they tended to feel tacked on and not really part of the main narrative.