It would seem that help text disappears once the user does not have write access to a particular field. We've gotten around this by setting access control to read/write for Anyone, then using form rules or ISF to make them read only. We use this any time we want help text to remain on the form during all moments. Hope this helps!
I am able to accomplish your use case in RC08 by setting the Access Control for the dictionary to View/Edit for the participant(s) who need to see the help text, and then setting the Display Properties to Read-Only.
yes its very strange. I set the field to TEXT and used both "Rules" and "ISF" to change the field to Read Only and its still invisible to the users. Maybe its inherant to the version that I am on 2008.3