Looks like loading the jQuery UI breaks the field. The only core object loading the UI seems to be the FileIframeField, and because that is used by the Image Field, you can't have an ImageField and the AutocompleteField on the same form. Since I can't see the UI being used (?) I just block it from the AutocompleteField::Field() method:

Requirements::block(THIRDPARTY_DIR . '/jquery-ui/jquery-ui-1.8rc3.custom.js');

A bit crude maybe, but eh... Maybe it's the Autocomplete plugin in the UI? Anyway, the only other object I saw using the UI is the DOM. Haven't tested it, since I don't use the DOM so much, but I thought I'd mention.

In case somebody is still trying to use the AutocompleteField in the ModelAdmin - i got it working and posted my solution here: http://code.google.com/p/ss-module-formfields/issues/detail?id=1