In reply to jussy:
i got a WD mybook network one. Fits all your criteria apart from your USB backup option and the fact you need to connect it via ethernet to your wireless router.
3TB storage, ethernet connection, does mac and PC simultaneously, supports multiple users, allows backup over wifi or ethernet connected to router, apps for ipad/ipod/iphone/android. Only thing I havent figured out is how to backup the android tablet to it.
Really easy to set up and users/priveleges managed through internet browser
Hope that helps
David