This is the title of the bug report I filed with the Apple Bug Report Tool yesterday. The problem ID is 6069931; I think you can only see your own bug reports in the tracker though, so you may not be able to see my report. FWIW, I filed it against Mac OS X 10.5.4 (the current version), with "Feature (new)" as a severity. For reference, the full text of the bug report follows:

11-Jul-2008 11:58 AM Benny Siegert:

I would like to kindly request the inclusion of mksh as an alternative shell into future versions of Mac OS X.

mksh (the MirOS Korn Shell) is a Korn Shell derivative under a BSD-like licence. It includes the features from the OpenBSD ksh as well as additional bash compatibility. Almost all bash scripts run under mksh, however with a speedup of about a factor 2. The code was checked for security problems by Coverity, Inc. UTF-8 support is fully integrated. With all these features, it is still very small, just 240 KiB as an i386 binary. The build process is done by a script called Build.sh, Mac OS is already a fully supported and maintained platform.

This could potentially replace the ksh93 included now as /bin/ksh or be installed side by side as /bin/mksh. In short, it can be regarded a more modern replacement for older ksh releases.

The homepage of mksh is located at http://mirbsd.de/mksh. The current version is available from https://www.mirbsd.org/MirOS/dist/mir/mksh/mksh-R35.cpio.gz.