I've wanted a libbitcoin since... well, since I first started browsing the bitcoin source code.
The consensus of the core bitcoin development team is to move towards a libbitcoin in small-ish, incremental steps, NOT to move to a full-blown API in one fell swoop.
The Bitcoin Consultancy folks disagree with that approach, and are moving ahead with a libbitcoin of their own that it rewritten from scratch, and I suspect there will be at least two or three other alternative implementations rewritten from scratch popping up over the next year or so. Which is why I'm spending a lot of time thinking about and working on cross-implementation testing.
(I'm supposed to be on vacation here in Australia, but I'll try to find some time to upload what I've done so far to github).