No it does not work. Are you certain it worked in 10.5 and 10.6? I was under the impression this never worked over the login screen. JAMF Helper is an app, not a binary like the jamf binary, so it needs a user to be logged in to run. Or, at least, with Lion Apple put additional restrictions on what is allowed to run while at the login screen.
Other tools may be able to accomplish this. In the past, some people used iHook, but that hasn't been updated since 2008 so I don't know that it works in Lion.
There are a couple of other options in Lion-
Someone suggested in one of the threads here (can't find it at the moment) that you can unload the loginwindow process. While this won't display a nice message, it will at least prevent someone from trying to log in. Reload it when the script or installation is done.
You could also write into the loginwindow plist and add the LoginWindowText key and a string with the text you want to display. Its not as "in your face" as the jamfHelper (and also won't prevent a login), but it at least gives them a message about what's going on.
