The testparm(1) man page says:

       If  testparm  finds  an error in the  smb.conf file it returns an exit
       code of 1 to the calling program, else it returns an exit code  of  0.
       This allows shell scripts to test the output from testparm.

Yet when I try running testparm on invalid files, it exits with 0.  E.g.:

       testparm /bin/cat

produces the following output (cut and pasted from another window:)

$ testparm -s -x /bin/cat
Load smb config files from /bin/cat
params.c:Parameter() - Unexpected end-of-file at: ELF
Loaded services file OK.
# Global parameters
[global]
$ echo $?
0
$
----end of cut and paste----

I see this on RH 8.0 with the following RH produced samba rpm's:

samba-common-2.2.7-4.8.0
samba-swat-2.2.7-4.8.0
samba-client-2.2.7-4.8.0
samba-2.2.7-4.8.0

and on Fedora Core 3 with the following RH produced samba rpm's:

samba-client-3.0.10-1.fc3
samba-common-3.0.10-1.fc3
system-config-samba-1.2.28-0.fc3.1
samba-swat-3.0.10-1.fc3
samba-3.0.10-1.fc3

I did check over at RedHat's bugzilla, but did not find this
bug reported there, either.