I took a look at the issue (reams of conversation) and the code (quite complex) and decided I'd rather build a simpler form builder targeted only at Bootstrap 3. So I've started a new gem: simple_bootstrap_form whose the goal is to mimic a subset of the Simple Form API, and layout only for Bootstrap 3. This gem can be used as a stopgap until the Simple Form folks finish their work. This gem is now powering the forms on this blog site.
The problem with Simple Form is that it is a generic framework that will layout forms for multiple different CSS frameworks (right now Bootstrap 2.3 and Zurb Foundation). The challenge with this is that greater flexibility incurs greater complexity in implementation. I believe the continued non-appearance of Simple Form Bootstrap 3 support reflects the difficulty people have in working in this more complex framework, and getting the results integrated and released.
While it is early days for this gem, it should be much easier to maintain and extend, given its single CSS framework target.