To sum up...
HD-only mode basically turns SMF+SD into a helpdesk, hiding all the boards, topics, and everything else; it just appears to be a helpdesk then. But it's a helpdesk that can leverage all of the bridges to other systems.
Compatibility is intended for 2.0 RC2+ only. Was one of the first questions the dev team asked itself - and answered - supporting two different sets of code is hard enough normally, but given that it would have to be a series of rewrites in order to work on 1.1.x as well as 2.0, we reasoned that running with 2.0 was a better route for us in the longer term.
Release date is when it's ready. The dev team have a roadmap in mind and we've talked about provisional dates for release to beta testing as well as provisional public release dates however we'd rather get you all a version 1.0 that is working and bug-free rather than meet a date on a calendar. If it's late, it's late because we wanted it to be awesome, not because of anything else.