Users that are members of 16 UNIX groups (maximum number of groups allowed for NFS) lose group access. Apparently default group is being counted twice. Access still works fine from UNIX shell.
Further testing on Samba 3.0.4 shows this bug has been fixed since 3.0.3.
fixed in 3.0.4
sorry for the same, cleaning up the database to prevent unecessary reopens of bugs.
database cleanup