According to an article published by TechCrunch, George Hotz the first person to come out with a Jailbreak for iPhone 3GS for Windows, has also come out with Mac compatible version. You can read more about the article here. You can also visit http://purplera1n.com/ to download the Windows or Mac version.

You can get more detailed information and step by step guides on how to unlock your iPhone by following instructions on a blog I follow myself: http://www.iphonedownloadblog.com/jailbreak/