As promised, Nate True - who yesterday released a Windows version of the iPhone jailbreak for 1.1.3 - has followed up with the release of a Mac jailbreak for 1.1.3. Like the Windows version of the jailbreak, it is required that you either start with a 1.1.1 or 1.1.2 iPhone or downgrade your 1.1.3 iPhone to one of these versions. When downgrading your firmware from 1.1.3, it is also recommended that you downgrade iTunes to 7.5.
The good news is, compared to the Windows jailbreak, the Mac jailbreak has considerably fewer steps - making it noticeably simpler. Either way, these jailbreaks aren't exactly what anyone would call a cake walk.
As always, proceed at your own risk.