The Shell Shock bash vulnerability that has been doing the rounds lately will never be patched on OS X Leopard as it is simply too old. Luckily Matt White has posted a fix for Debian Lenny and as Leopard and Lenny use the same version of Bash (3.2) the same fix can be applied. I've tweaked his update script for Os X (as by default there is no "wget" or "seq" (sequence) so simply download it from here, mark it executable (chmod +x) and run it to upgrade the version installed to 3.2.54 which includes the needed patch.
Your old version will be saved as /bin/bash.broken and the new version will be installed into /usr/local/bin/bash and a symlink created from /bin/bash to it Updated: The script now simply overwrites the old version as having it hanging around could be considered dangerous. If you want to keep a copy then you'll need to manually copy it somewhere safe (eg /var/root/). The script also now removes /bin/sh and creates a symlink to /bin/bash. Once installed simply log out and back in again and you'll be using the new version, and any CGI-BIN scripts you use will also be safe (well, at least from that bug!).
Happy patching and let's hope that it is a long time before a vulnerability like that crops up again.
UPDATE: A few people have asked if they can have a copy of the binary as they don't have the build tools installed. The latest version can be downloaded from here (PPC ONLY! I don't have an Intel install of 10.5 sorry), once downloaded you'll need to mark it executable and copy it to /bin/bash
UPDATE 2: I've just had pointed out to me that /bin/sh on Leopard is actually a copy of /bin/bash, not a symlink so calling /bin/sh will still leave your system vulnerable. To get around this either copy /bin/bash to /bin/sh or better yet symlink /bin/bash to /bin/sh so updates to bash will also result in /bin/sh being updated. Finally, I've changed the script to simply replace /bin/bash rather than install into /usr/local/bin/bash and symlink.
UPDATE 3: And another patch is out - binary has been updated and is available here or for those with the build tools installed you can compile from source using this script. Hopefully this is the last for a while as it addresses all currently outstanding CVEs.
Since the release of the Shell Shock bug for Bash I thought I'd compile all the fixes I've done for OS X into my updating OS X Leopard for the modern world document as I'm now running a newer version of OpenSSL, PHP, ClamAV etc. The old doc (using php 5.3) can be found here though as PHP 5.3 has been discontinued you should no longer refer to that document.