This hint pointed out that this older hint for running Automator workflows from AppleScript was broken.
But if you saved your Automator workflow as an application rather than a workflow, then used AppleScript to open it, it would run. Although I did not see this as a real hint as opposed to an obvious behavior, it was the only way to get Automator workflows to run via AppleScript -- until now. Coming back to this on a whim, it seems the 10.5.1 update has fixed this problem, and the older hint now works correctly. For example...
tell application "Automator Launcher" to open "Macintosh HD:Users:userNmae:Desktop:foo.workflow"
...works as expected under 10.5.1.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20080128102421712