as you can see, my version of md5sum does not support --string either

/usr/bin/md5sum --help
Usage: /usr/bin/md5sum [OPTION]... [FILE]...
Print or check MD5 (128-bit) checksums.
With no FILE, or when FILE is -, read standard input.

  -b, --binary         read in binary mode
  -c, --check          read MD5 sums from the FILEs and check them
  -t, --text           read in text mode (default)

The following three options are useful only when verifying checksums:
      --quiet          don't print OK for each successfully verified f
+ile
      --status         don't output anything, status code shows succes
+s
  -w, --warn           warn about improperly formatted checksum lines

      --strict         with --check, exit non-zero for any invalid inp
+ut
      --help     display this help and exit
      --version  output version information and exit

The sums are computed as described in RFC 1321.  When checking, the in
+put
should be a former output of this program.  The default mode is to pri
+nt
a line with checksum, a character indicating type (`*' for binary, ` '
+ for
text), and name for each FILE.

Report md5sum bugs to bug-coreutils@gnu.org
GNU coreutils home page: <http://www.gnu.org/software/coreutils/>
General help using GNU software: <http://www.gnu.org/gethelp/>
For complete documentation, run: info coreutils 'md5sum invocation'


/usr/bin/md5sum --version
md5sum (GNU coreutils) 8.13
Copyright (C) 2011 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gp
+l.html>.
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.

Written by Ulrich Drepper, Scott Miller, and David Madore.
[download]

but it seems there was a version that did http://www.linuxdevcenter.com/cmd/cmd.csp?path=m/md5sum

You may wish to see http://askubuntu.com/questions/53846/how-to-get-the-md5-hash-of-a-string-directly-in-the-terminal for workarounds.