Ok, now that a new version of QuickPwn is out for Mac OS X, you don’t have any excuse for not jailbreaking your iPhone 3G. I already posted a QuickPwn guide a couple weeks ago called QuickPwn Mac OS X Tutorial. The method is exactly the same so instead of doing it all over again, I’ll just put the link to the QuickPwn for Mac tutorial.
Here it is: QuickPwn Mac OS X Tutorial
And here is the PwnageTool 2.1 for Mac OS X Guide