I have the same problem -- the permissions actions work fine for Active Directory usernames but not for Forms-Based Authentication usernames. We have a MOSS environment with both AD and FBA (LDAP in our case) authentication.
My suspicion is that the colon that SharePoint includes in the FBA username (our FBA usernames look like "LDAP:firstname.lastname@example.org") is the culprit. That colon is part of a prefix generated by SharePoint that is added to all FBA usernames.
Using SharePoint Designer's "Build a dynamic string" action, I've tried stripping off the "LDAP:" prefix of the username and using the result as the username in the permission action, in which case no error is generated, but
the permission doesn't get changed for the user. I've tried looking up the user's Work e-mail property and using that as the username with the same result (no error but no permission change). I've also tried subsituting the Hex value (%3A) for the colon,
but that generates an error. I've even tried using the GUID of the user as the username, but that generates an error too.
Has anyone had success with FBA usernames?