I've edited a couple of questions today on the main Webmasters site for re-tagging, and the field is no longer providing the suggestions drop-down. As a quick test to make sure I wasn't expecting something that never happened, I tried over on the main SO site, and it does work over there.
This is edit-only, as far as I can tell. The suggestions drop-down does appear when authoring a new question.
Didn't think to check the console previously, for some reason. Firefox 4 reports:
Error: bindTagFilterAutoComplete is not defined Source File: [question URL] Line: 2