PHP trouble with concurrent sessions and AJAX
The assumption was wrong.
...session data is locked to prevent concurrent writes only one script may operate on a session at any time http://us.php.net/session_write_close
In other words, you could be running 100 AJAX requests for a user at the same time and they would each wait their turn.
My problem turned out to be an error in selecting the right session table column resulting in my sessions being re-created each load.