Geohot, the developer of Limera1n jailbreak for Windows, has now released the same tool for Mac OS X platform. Limera1n for Mac tool is mere 478kb download from http://limera1n.com. Now even Mac users would be able to jailbreak following devices:
-iPhone 4, iPod touch 4G, iPhone 3GS, iPod touch 3G and iPad
All these devices must be running iOS 4.0 to iOS 4.1 update. Download limera1n app on Mac OS X. Start it and follow the instructions in the box.
Just in case you face any issue, do let us know so we can help you fix it or at least escalate it.