Not long after the Windows version of iPhone 1.1.3 jailbreak (by Nate True) was thrown in the wild, the Mac version too made its appearance on the web. It's already available for download, but keep in mind that this particular jailbreak may be hazardous to your iPhone, as the cre.ations.net themselves note in the statement of risk.

As all upgrades are risky, this one is doubly so. You may have to restore your phone using iTunes and start again if it fails. Make sure to back up first!

Backup can be performed on iTunes, while 1.1.3, just like like the 1.1.2 jailbreak, comes as an upgrade which means that you need to have a 1.1.1 or 1.1.2 jailbroken phone already, before you can begin.

If you're the owner of an unlocked phone, it seems to make little to no difference, since an update on the cre.ations website confirms that unlocked phones appear to remain unlocked and work properly after the update. This information is reportedly scattered across several reports.

You'll need to start with a 1.1.1 or 1.1.2 jailbroken phone with Installer.app installed,Plug in your phone via USB to your computer, download, extract, and run iBrickr Special 1.1.3 jailbreak edition (ibrickr.exe in the archive). iBrickr will confirm what firmware you are running on your phone, thus making sure you can run the update.

Next, iBrickr will start guiding you through the process of obtaining and modifying the 1.1.3 firmware, uploading the new firmware image to your phone. iBrickr should do its job in about 10 minutes, after which it would allow to finally run the installer for the "1.1.3 soft upgrade" package.

That as far as we go with cre.ations.net's how-you-do-it, the rest you can check out for yourselves. Remember that this particular jailbreak may be hazardous to your iPhone. If you proceed with installing it, you're doing it at your own risk.