Another bug landed on my foot. This time, some AJAX combo boxes weren't updating on the development server, but worked fine on my machine. Fiddler showed that the ScriptResource.axd "files" were 404ing, so it looked like a problem with the application setup.
There was also a script error: "Sys is undefined".
It turned out that the AJAX settings were missing from web.config. Adding them in made the scripts run correctly.