Struggling with nginx, UJS and asset server

Sure this is a dead simple one, but I’m struggling with the right config
for nginx to enable to requests for the UJS (Unobtrusive Javascript) to
pass through to the mongrels. Worked all fine and dandy using Ezra’s
latest nginx config file, until I enabled the
action_controller.asset_host in the production environment (I have nginx
serving the static files, passing the other requests through to a
mongrel_cluster on another box).

What seems to be happening is that nginx is trying to serve up the UJS
js file (and sending a big fat nothing), rather than passing it on.

Any pointers?