He shows that right now, there are various differences between browsers and AT's handling of ARIA in trees, so it's not very stable right now. But he does have a best-choice example which bascially works with current browsers and screen readers.
In my opinion, VoiceOver not keeping the cursor in check with keyboard cursor and moving tabIndexes is indeed a bug. I would think this, combined with webkit's in-page link bug would make thing pretty hair-pulling for Mac users.
A great read, although a lot to take in. Although I've never been interested in trees like that, this gives a good insight into how ARIA works to make the web more accessible. Thanks for keeping us aware of this stuff, poes.