In a recent interview on html5doctor by Bruce Lawson of new Doctor Steve Faulkner, someone asked why HTML5 document outline wasn't being implemented as expected.

Quote Originally Posted by stevefaulkner
My understanding is that the outline algorithm as currently defined is not intended to be implemented in browsers. Meanwhile it has sort of been implemented by the JAWS screen reader, but in a borked way. It is a proposed feature that implementers in general have not shown an interest in. for this reason it is a feature at risk in the HTML5 CR specification and will likely be dropped before it becomes a Recommendation, it will probably live on in HTML 5.1 as a proposed feature and we will see how it goes.

So the outline is at risk and developers should be wary of using it. Also ran across this tweet from Steve:

He mentions JAWS can/will announce each region and if you've got hundreds of them they get tedious :P