Thanks for your continued patience. I did realize that I was using the wrong URL (I was appending the token to /GPServer rather than GPServer/Extract Data Task. However, fixing that didn't change the behavior. The default code doesn't work, the second set of code you provided results in an "invalid token" message, and in the third set of code firebug shows that the correct URL is retrieved successfully, but the application stops after the page is called and there's still no checkboxes of data layers. Incidentally, the most recent code (which eliminates the ?f=json) also breaks the widget for an anonymous service for me, whereas the other sets of code worked fine for an anonymous service.
Perhaps I should wait until I have a public-facing AGS10 server (it should be ready in just over a week) so that I can create an example service with token (but non-sensitive data) and post it here? thanks again for your help,
Jon