...

...
        cmd = "pvdisplay %s | grep -q 'planetlab'" % lvm_partition
        already_added= utils.sysexec_noerr(cmd, log)

        if already_added:
            log.write( "It appears %s is part of the volume group, continuing.\n" %
                       device )
....

fails because pvdisplay returns a non-negative number on error

[root@mlab1 ~]# pvdisplay /dev/sdb1
  Failed to read physical volume "/dev/sdb1"
[root@mlab1 ~]# echo $?
  5