I think I read that it'll require publisher enrollment. And it will require online service, I suspect you'll need a Live gold account. You won't be able to just throw you 360 discs in the One and play. There will be a download to be done or some such thing.
IGN has a note about it: http://www.ign.com/articles/2015/06/15/e3-2015-xbox-one-backward-compatibility-update-enables-360-xbla
Electronically purchased games will show up in your library automatically. Discs will require a download. Developers and publishers need only approve what games are eligible. Microsoft apparently takes care of the rest. No word in the note about whether a paid account with Microsoft is required.
Microsoft has a note too: http://www.xbox.com/en-US/xbox-one/backward-compatibility/
Says it's available at no additional cost, but I don't know if that assumes you already have a paid Live account? (My bad, further down on the Microsoft page it indicates a Gold account is not required!)
Looks like it's up to Bethesda on this one.