Have a fairly straight-forward D6 site except that it's set up to have an admin. which has it's own settings file, for the purposes of using https, not using memcache, and having a separate cookie...everything else including the db is set the same.
Created a new module with a menu callback of api/mymodule/ajax which returns html that is used to add another row of teasers to a display. Calling it in .click() via .ajax()
On my local system, it all works fine, whether www. or admin. When it's moved to qa servers, admin. works fine but www. returns a 301 and redirects to the front page. Can't think of anything (other than something deliberate) that would have the new callback (whether via the link or entering it in the browser) result in a 301 in one case but not the other.